Tricks to get better MPG
April 29, 2008 8:21 AM   Subscribe

Simple tricks to improve MPG? For instance, I've thought about removing the rear seats/internal molding to lighten my car. I've also thought about aero-improvements, such as moon-disc wheel covers, or even somehow making a cover over my rear wheel wells. I'll be removing my rear wing this weekend, but what other tweaks could I do?

Other things: sealing off the "styling" holes in the front fascia, slightly over inflating tires, etc...
posted by raikkohamilonso to Travel & Transportation (39 answers total) 12 users marked this as a favorite
 
I don't know if you're interested in behavioral changes, but here you go:

- Slow, gradual accelerations from stop.
- Drive at 55 on the highway for maximum fuel efficiency.
- On uphills, focus on keeping your RPMs constant, not your speed.
posted by dondiego87 at 8:23 AM on April 29, 2008


Oh, and that's 55 miles per hour.

Misc. links I remembered about:
- 10 Ways to Boost Your MPG (Consumerist)
- Crazy story about "hypermilers", intrepid and potentially dangerous souls who do whatever it takes to maximize their fuel efficiency using behavioral changes only.
posted by dondiego87 at 8:26 AM on April 29, 2008


If you drive a stick, you can coast on highway downhills. In theory, you're putting more waer on your throwout bearing, but I've not yet had that problem. Doing just this, only on my afternoon commute (25 miles - in the mornings I'm not awake enough to pull it off) ups my MPG from 30 to 33, approximately. Mind you, this is in a '90 CRX Si (sport model), lowered on account of broken springs (don't take a CRX off-road in Glacier National Park), with a roof rack and enough rust to affect the drag coefficient.

Also: on big lights that you know take a while, turn off the car. If your car takes a lot of cranking to get going, this is obviously a bad idea, but if yours is like mine and only needs one rotation of the engine to get going, it works pretty well. I have to pay better attention to the cross traffic to avoid being honked at, but I can usually be off the line, including starting the car, putting in the clutch, putting it in gear, and letting out the clutch, before the driver in the other lane. I've just started experimenting with this as a general gas-saver.
posted by notsnot at 8:32 AM on April 29, 2008


Car Talk's guide to better fuel economy (skip to parts 2 & 3)
posted by Koko at 8:44 AM on April 29, 2008


Make sure your tires are properly inflated. If driving 55 mph on the freeway will get you killed, go 60 mph. (WRT this, I really am going to practice what I preach. Yes.)
posted by rtha at 8:49 AM on April 29, 2008


I find it very, very hard to believe that any of the external changes you mention (new wheel covers, filling in the styling holes) are going to make much of a change when compared with changing your driving habits. Maybe removing any large spoilers would. In the absence of strong, quantitative evidence (from a disinterested party, not from someone selling "high efficiency" parts of some sort), I would not expend any effort on minor modifications to the outside of your vehicle.

The tire inflation pressure can make a noticeable difference, but if you run them too hard, you'll just end up wearing them faster, and eat up any gasoline savings on new tires. I've heard anything over 3-5 PSI beyond the manufacturer's specs is a bad idea.

The biggest thing you can do to save gas is to slow down. Not only reduce your top speed on the highway, but also accelerate slowly and smoothly from standing stops, without letting the engine rev too high in each gear. Anticipate when you may have to stop and take your foot off the gas. Don't keep your foot on the gas right up until you slam it onto the brake.

Don't do anything unsafe, but realize every time you touch the brake, you're converting precious, precious gasoline into waste heat. If you can anticipate future stops and let the car coast to a halt or near-halt without much braking, you'll save gas. (And if you do get stuck in traffic, trying to roll at a slow-but-constant speed is preferable to lurching along, three feet behind the car in front of you. However, people may just cut you off whenever you leave space, so YMMV.)

In my car, the fuel-economy difference between aggressive driving and very passive driving is almost 10MPG (19/20 MPG when driven aggressively, to 29/30 if I'm very careful). That's a huge difference. Part of it is due to my car having a turbocharger, which makes hard acceleration the equivalent of pressing the flush handle on a toilet, but you'd probably see pretty substantial differences in any car. Nothing you're going to do to the car's exterior is going to have returns like that.

Make sure you're getting all the low-hanging fruit first, before you start spending money and effort on the last tenth of a percent.

Also: bicycle.
posted by Kadin2048 at 9:01 AM on April 29, 2008 [1 favorite]


I own a hybrid that has a realtime MPG gauge, and I reset the trip MPG every day, so I know what affects my MPG pretty well. Some of the stuff only applies to hybrids, but here are my personal experiences (YMMV, literally!):

- Stoplights absolutely kill MPG. My car even shuts the engine off at stoplights most of the time and runs off the battery until I accelerate again, but even then I can get 5-7 MPG better if I have no stops versus several stops for the same distance over an hour drive.

- Heating and AC drop MPG by around 3-5 MPG.

- Anything over about 55MPH drops the MPG a few points, and once you get up over 70 it can drop 10+ points.

- Weight doesn't seem to matter as much as you would think. I had so much weight in mine one time that I was afraid I was over the weight limit for the car, and the MPG was down only 2-4 points if anything.
posted by burnmp3s at 9:03 AM on April 29, 2008


Oh, and keep your intake air filter clean. That can adversely affect gas mileage, especially if you drive in a dusty area. Mine gets full of pollen in the spring; I normally just shop-vac it out and then have it replaced as part of normal maintenance.
posted by Kadin2048 at 9:05 AM on April 29, 2008


1. Combine errands.
2. Watch traffic and lights well ahead, and pace your driving accordingly.

Each time you brake, you're wasting fuel. It's amazing how many people race up to a red light only to apply their brakes.
My brake pads last forever, and that means I'm saving pads and fuel.
posted by weapons-grade pandemonium at 9:06 AM on April 29, 2008


Don't forget that you need to keep routine maintenance up. Fuel filter, spark plugs, oxygen sensors, etc. If any of those are old, they're going to adversely affect your mileage.
posted by pete0r at 9:20 AM on April 29, 2008


Another article on testing common recommendations: bottom line as many suggest is that better driving practices is far away the winner. I can't track it down but I remember listening to a report on public radio a year or so ago where various fuel-economy tactics were tested, and again, savings from changes in driving habits (generally, less aggressive driving that minimized acceleration and braking) dwarfed technical improvements.
posted by nanojath at 9:26 AM on April 29, 2008


Wait, running the heater costs gas mileage? I get that A/C does, but that heat that's generated by the engine anyway, right?
posted by luser at 9:57 AM on April 29, 2008


Seconding the keep-constant-speed-in-stop-and-go-traffic:

SOMETIMES ONE DRIVER CAN VASTLY IMPROVE TRAFFIC
posted by toastchee at 10:08 AM on April 29, 2008


Running the fan when heating will increase the load on the alternator, which might drop you a few HP. It won't be nearly as drastic a power decrease as running the A/C.
Most cars are heated by engine coolant running through a heater core behind the dash. When you switch your heat on, a diverter valve starts circulating coolant through this secondary radiator. You don't need to be shy about running your heat.
posted by tmt at 10:15 AM on April 29, 2008


Job no. 1 is SLOOOOOOW DOOOOOOWN

The institution of 55MPH speed limits on the highways was about saving energy, not lives.
posted by DoctorFedora at 10:27 AM on April 29, 2008


If it's not fuel injected, you can pinch the jets. Consult a knowledgeable mechanic about the downsides to this. But we did it on a pickup with a mid 70's Chevy 350 and got reasonable performance and about 25 mpg.

Depending on the vehicle, slightly larger tires might help. Tires with an extremely hard compound also tend to have low rolling resistance.

Keep in mind the safety issues with those last two suggestions.
posted by krisak at 11:05 AM on April 29, 2008


Keep the windows closed while driving.

You could try to DIY a wheel well skirt or ground effects to make the car more aerodynamic.
posted by JJ86 at 11:20 AM on April 29, 2008


Leave an adequate "space cushion" in front of you, so you can decelerate
using the throttle, not the brakes.

Get your ego out of the driver's seat. It's not a race, no matter how
much it looks like one.

As a longtime passsenger, I notice that most drives will increase their
speed 1 to 3 mph when they are passed by another vehicle. When
there are no vehicles within some "magic horizon" before them, they
lose these 1 to 3 mph. This small variation in speed is especially
noticeable on freeways.

In a heavy vehicle this small acceleration and deceleration can have an
effect on mileage.
posted by the Real Dan at 12:57 PM on April 29, 2008


If you're after Mp$ rather than Mpg, then filling up at the coldest time of day (ie late night/early morning) can save you evaporation and expansion losses.

- Weight doesn't seem to matter as much as you would think. I had so much weight in mine one time that I was afraid I was over the weight limit for the car, and the MPG was down only 2-4 points if anything.

I think this is probably a hybrid thing - more weight takes more energy to get moving, but a hybrid would get most of that loss back by getting extra power from the regenerative braking when slowing that extra mass back down. In a standard car I imagine the energy loss due to extra weight would be higher.
posted by -harlequin- at 2:59 PM on April 29, 2008


Drive more smoothly! I haven't witnessed it in a hybrid, but I have seen cars that have variable displacement (some cylinders are shut down to improve MPG) become completely useless and confused by stop-go-stop-go driving. And I don't mean stop-go at traffic lights, I mean when you're trying to maintain 60 MPH so you press the pedal down until you're going 62, let off the pedal too much and go down to 57, press, release, press, release, not only is it horrible to sit through but it's horrible for your car.

I doubt, since you're asking this question, that you drive like this, but I just wanted to make this note in case other people are reading the answers. It's seriously bad driving, bad for fuel economy, and makes traffic worse.


As far as aerodynamics go, this guy claims to have gotten another 30 MPG by making his Civic incredibly ugly. I mean, aerodynamic. In general the whole ecomodder site seems like it could help you out. As far as weight goes, the superlight Loremo LS gets a claimed 154 MPG. So you do seem to be on to something.
posted by blacklite at 4:47 PM on April 29, 2008


raikkohamilonso posted "I'll be removing my rear wing this weekend, but what other tweaks could I do?"

FYI: Not all wings increase drag, the factory spoiler on the Fiero actually reduced drag by 0.01. Also the new beetle can see a massive improvement in gas mileage with the right spoiler.
posted by Mitheral at 6:54 PM on April 29, 2008


Wait, running the heater costs gas mileage? I get that A/C does, but that heat that's generated by the engine anyway, right?

The Car Talk guys addressed this question recently. I don't remember if it was in their newspaper column or their radio show. But as I recall, their conclusion was, they weren't sure.
posted by Dec One at 7:11 PM on April 29, 2008


I can't believe how many people race to a red light, it's insane! I usually just shift to neutral since I drive manual and coast and anticipate the light turning green since I usually drive the same route to work.

Also I've heard that Cruise control lowers MPG but I don't understand why? Isn't it the same as keeping your foot steady on the gas to maintain the same speed? Am I missing something?

Also what decreases MPG more, driving with the windows down or with the AC on and windows up?
posted by spacesbetween at 8:04 PM on April 29, 2008


As far as aerodynamics go, this guy claims to have gotten another 30 MPG by making his Civic incredibly ugly.

That's obviously rubbish. 3, maybe. 8? maybe. 30? Bullcrap.

Driving habits are by far the best, safest, and most sensible ways to do this. Modifying your car will not at all be cost effective, and removing the rear wing may not only be making it worse, it may actually be dangerous - what car is it? Rear spoilers (wings have airflow on two sides, spoilers on just one) usually reduce drag and give better high speed stability.
posted by Brockles at 10:37 PM on April 29, 2008


according to the great Jeremiah Clarkson, coasting in neutral uses more fuel than leaving it gear. So slow 'engine' breaking up to lights is better than shifting into neutral, as the momentum of the car keeps the engine running with minimal use of fuel. I drive a european diesel car so I get 50mpg without much trying. I guess buying a different car will make the most difference. 25-30mpg! yikes?
posted by sdevans at 4:28 AM on April 30, 2008


according to the great Jeremiah Clarkson, coasting in neutral uses more fuel than leaving it gear.

I love Top Gear as much as the next chap but I find this very hard to believe. Let's not underestimate Clarkson's capacity for cognitive dissonance. This is a person who, notwithstanding almost all of the world's credible climatologists saying to the contrary, doesn't believe that climate change is man made.

I would treat that assertion that coasting in neutral is more fuel efficient than gear breaking with great caution. I'm no engineer but it seems counter intuitive. That said, if anyone has a credible link to this effect, I'll be happy to row back from this position. We have considered this point and the consensus appeard to be that coasting downhill reduces fuel consumption.
posted by dmt at 7:04 AM on April 30, 2008


No, it's absolutely true, no matter how much of a cock Clarkson is. In neutral, the car still allows fuel to be fed to the engine - enough to maintain idle on the normal idle circuit. On throttle off deccel, the fuel is cut completely to produce engine braking.

The only time it may shift in which is better is on long downhill sections (gently sloping) where engine braking would be impossible to maintain. The distance of the coast then becomes a factor in which is more efficient, but up to 'the longest distance the car will engine brake before needing to be accelerated again" then leaving it in gear is uses no fuel at all.
posted by Brockles at 7:24 AM on April 30, 2008


Good god, that Askme you link to is a torrent of lack of knowledge and duff physics/car knowledge...
posted by Brockles at 7:28 AM on April 30, 2008


dmt: There is confusion because once upon a time, not too long ago, engines used fuel when coasting in gear. But today, everything is computerised, and if the foot is off the accelerator, and the engine (due to being in gear) is being turned sufficiently fast by the car's momentum that it can't stall, then the computer realises there is no need to inject fuel to keep the engine idling, and so doesn't.

So leave it in gear, unless you've got an old car.
posted by -harlequin- at 9:16 AM on April 30, 2008


The cooling system of your vehicle, which includes the openings in the front fascia so that air can get to your radiator, were designed for a worst-case scenario. Think towing a trailer in Death Valley in the middle of summer. Begin blocking off part of your grill openings, like a couple inches at a time. Keep enough airflow to avoid overheating, but no more.

Look at the underside of your vehicle and see if you can install a belly pan. It's a custom job, but sheet aluminum is cheap. This will smooth out airflow under your vehicle, which reduces drag.

If your vehicle is a 1996 or later model, invest in a ScanGauge II. I got one for my 2000 Protege and I regularly beat my highway MPG rating in city driving by at least 10%. When I first got mine I had it set to display real-time fuel economy. I've since switched the display to show my daily average. It's like a game, trying to get the best fuel economy during my commute. My best is 32mpg on the way to work, and my car is rated at 22/28.

Coast up to red lights. Coast as much as you possibly can, actually. Let people pass and laugh at them for wasting gas. Take corners briskly, but not unsafely, to avoid having to re-accelerate. Accelerate gently going downhill and coast a little up the next hill.

Keep your tires inflated to the proper pressure. Have a four-wheel alignment done. Switch to synthetic oil for the engine, but watch out for oil leaks suddenly appearing after you switch. Engines use more fuel when cold, so install an engine block heater and use it every night - this way the engine gets up to normal operating temperature much faster, which saves gas.

Don't exceed posted limits, and drive below them when you can. I regularly drive 60 in a 65 zone on our local interstates, but only if traffic allows.

Hope this helps.
posted by eratus at 3:46 PM on April 30, 2008


The cooling system of your vehicle, which includes the openings in the front fascia so that air can get to your radiator, were designed for a worst-case scenario. Think towing a trailer in Death Valley in the middle of summer. Begin blocking off part of your grill openings, like a couple inches at a time. Keep enough airflow to avoid overheating, but no more.

Be sure to test this extensively in a wind tunnel to ensure that you are not actually creating more drag than 'streamlining'. You will need to better the free flow through a radiator matrix and it's ensuing removal of high pressure from the frontal area of the car that has already been considered by the manufacture during years of development. Also, be aware that removing airflow in this manner may well cause medium to high speed instability as airflow is redirected around the car in ways that was not envisaged by the manufacturer when their (much cleverer than this suggestion) designers defined a stable aero platform.

Or, of course, go nuts with a tape gun, risk damage to your engine if you get it wrong and potentially create an aerodynamic stall over your rad at certain speeds* and perhaps just make it worse.

Look at the underside of your vehicle and see if you can install a belly pan. It's a custom job, but sheet aluminum is cheap. This will smooth out airflow under your vehicle, which reduces drag.

Again, this may well utterly screw up the existing aerodynamics and cooling of your car. Be sure to test all the load conditions in the wind tunnel to ensure no hot spots in the engine bay or issues with cooling from this modification. Also, Ali is not all that cheap, and installation will be expensive and a lot of work if you want to do it properly. There are not "bolt your bit of 'mpg saving' plate on here" tabs underneath your car.

Keep your tires inflated to the proper pressure. Have a four-wheel alignment done.

Good advice. The only bit. Don't over inflate, though (stay within manufacturer/tyre specification) limits as you can seriously affect safety with this parameter through reduction of contact patch and ensuing grip.

Switch to synthetic oil for the engine, but watch out for oil leaks suddenly appearing after you switch.

Oil leaks? What? How random. There is no link between changing oils and sudden leaks in your engine.

As you may be able to guess, there is an awful lot more to 'taping up some ducts' and modifying your car, and any new (ie less than 10 years old) car has had massive amounts of work done on drag reduction and stability in wind tunnels. Don't mess with bodgy backyard solutions unless you are prepared to risk serious safety issues through high speed stability issues, or potential engine damage through unexpected cooling airflow behaviour.

Modify your driving behaviour and service your car regularly. These are the only sensible (and coincidentally most effective) methods for improving MPG without doing extensive research and work.

*I was part of the testing team for a high profile luxury car manufacturer that had a styling department-designed front spoiler that caused a total loss of cooling air to the intercooler radiators above a certain speed that resulted in a total engine meltdown. I know of which I speak... Yours won't be anywhere near as expensive a failure, but it is certainly possible.

Messing around with modern cars is something that seems to produce an awful lot of misguided, wrong, clueless advice. Your car is much more complicated than you think in how it is designed. Messing with it too much without understanding what you are doing is at best a game of diminishing returns, and at worst counter-productive and potentially damaging/dangerous.
posted by Brockles at 4:05 PM on April 30, 2008


go nuts with a tape gun

Right, this is exactly what I suggested. Nevermind the part where I said, "a couple inches at a time." I obviously meant 'go nuts.'

There are not "bolt your bit of 'mpg saving' plate on here" tabs underneath your car.

Which is why I didn't bother saying, "It's a custom job." Oh, wait...

Be sure to test all the load conditions in the wind tunnel...

I'm sure that, just like you, everyone reading this has access to a wind tunnel and knows how to interpret the results of same. Also, automakers never, ever err on the side of caution when it comes to the various subsystems of your vehicle. Ergo, there cannot possibly be room for improvement in the aerodynamics of your vehicle. I'm sorry for making such a silly assumption.

Good advice. The only bit.

Gosh, thanks for completely discounting everything else I said, even though everything I suggested is based on my own real-world experience that the experience of several other drivers I know. The ScanGauge II in particular is well-known for cutting fuel economy in half. I'm surprised you didn't share that bit.

There is no link between changing oils and sudden leaks in your engine.

I have seen several high-mileage engines develop leaks from seals after being switched to synthetic oil. Synthetic oil can squeeze out of much smaller gaps than petroleum-based oil. But again, thanks for completely discounting my own personal experience. It's comforting to know that there's someone like you out there willing to set me straight about my 20+ years of experience working on and modifying cars.
posted by eratus at 5:57 PM on April 30, 2008


Which is why I didn't bother saying, "It's a custom job." Oh, wait...

The OP asked for 'simple tricks', not 'expensive and potentially pointless modifications'. A flat floor is not necessarily better aerodynamically than the pressed lower surface of a car floorpan, and 'streamlining is always better' is a concept that was discarded in the sixties.

I'm sure that, just like you, everyone reading this has access to a wind tunnel and knows how to interpret the results of same.

That was precisely my point. Just 'taping up the front of the car until just before it overheats' will not necessarily improve drag one bit. To assume so is missing over half of the considerations of aero aspects of the car. It may well have precisely the opposite effect, and may create engine damage in the process of experimentation. It's silly, badly researched advice that ignores more parameters than a simplistic understanding of 'drag'.

I do have reasonable experience in wind tunnels, and with car aerodynamic development in general. Assuming just 'taping over the holes is good' or 'make the underside smooth is good' is spurious advice, and I felt it needed calling out. There is so much duff advice on these automotive threads that it really needs addressing. Urban myth runs rife through these threads more than any other that I have seen here, and clearly not that many people are in a position to address them.

I have seen several high-mileage engines develop leaks from seals after being switched to synthetic oil.

You have most likely seen several high mileage engines develop leaks. That's all. Engines develop leaks eventually, it's pretty much inevitable. I'd put large sums on the oil change being entirely coincidental, or due to additional work done at the same time. If your replacement oil 'squeezed out of smaller gaps" then it was a different grade oil, not a synthetic equivalent. The advantage comes with its ability to resist breaking down under load, not through being 'thinner'. Using the wrong grade oil could allow leaks to be more apparent, yes, but not cause them. Although that is unlikely in itself in a engine of less than 20 year old design.

It's comforting to know that there's someone like you out there willing to set me straight about my 20+ years of experience working on and modifying cars

It's disturbing that you are offering such misguided advice after that experience, to be honest.
posted by Brockles at 8:37 AM on May 1, 2008


You remind me of another person I encountered online several years ago. I was driving a Geo Metro and living in Seattle at the time. This other person owned the same year Metro I did, though I don't know where he lived. Someone had asked what kind of mileage we all got with our cars. I responded that I got a consistent 41mpg with my Metro. This other person responded to me: "You're wrong. I get 48mpg with my Metro."

I was not 'wrong' just because he got better mileage with his Metro than I did with mine, yet this was his attitude and he wouldn't budge from it. Most likely he merely lived in an area with fewer hills and less traffic than I had to deal with in Seattle. This assumption is supported by the fact that the fuel economy of my Metro increased to 47mpg after I moved to NC, where it is indeed much flatter and there is substantially less traffic.

Just because your experience is different than mine does not make me wrong, and I take offense at your 'misguided' remark. I am personally acquainted with several people that have successfully blocked off all or part of their grilles, are getting several MPGs better fuel economy, and have suffered absolutely no overheating effects, even in summertime. Are they all 'misguided' too? Is that 'bad research', even though it's working for them?

And about engines leaking after being switched to synthetic oils: nine different engines, same weight oil as the petroleum oil it replaced, and no other work was done except for the oil and filter. Nine engines, all of them leaked afterward. Further, switching back to petroleum oil stopped the leaks. These cannot all be coincidences.

I repeat, just because your experience is different than mine does not make me wrong. Further, aside from 'driving habits,' you haven't offered one single bit of advice to the OP. All you've done in this thread is criticize what others have had to say. You are being incredibly closed-minded, not to mention condescending.

To raikkohamilonso: There are a couple online forums you might want to check out. GasSavers.org and EcoModder.com. Both are dedicated to getting better mileage out of your car. Several members of those boards have accomplished quite a lot with their cars.
posted by eratus at 3:36 PM on May 1, 2008


Further, aside from 'driving habits,' you haven't offered one single bit of advice to the OP.

Driving habits, correct pressures and servicing of your car are the main things that will help the OP. Little else should be attempted without more in the way of knowledge - if they were able to fit and test aero modifications safely, they wouldn't need to be asking here how to do them.

Messing around with your cars aerodynamics is dangerous without proper testing, and just as likely to be counter productive without knowledge. It is NOT blanket 'good advice'. Removal of the rear spoiler (for instance) is likely to produce high speed instability or susceptibility to cross winds. Audi had a factory recall for the TT model, for example, to fit such a spoiler to reduce these effects. Removing the spoiler on a TT is consequently dangerous. It is unlikely that removal of the spoiler (assuming it is not a full wing) will actually reduce drag. That is some of the good advice I have been giving - to not believe urban myth/first perceptions, and that aerodynamics are more complicated than being assumed here. In the same way, covering over the rear wheel arches may produce braking or stability issues - it is not as simple as the (previously mentioned) sixties concepts of 'blocking all holes up is better'.

On the same line, taping over grilles and affecting airflow through engine bays and underneath cars (especially with flat undertrays) are areas that shouldn't be approached without proper understanding and testing - to ignore this is to risk potential accidents and damage to the car, Advice NOT to do something (with reasons given) is as good a piece of advice as any. Do's and don'ts are just as valid.

I am personally acquainted with several people that have successfully blocked off all or part of their grilles, are getting several MPGs better fuel economy, and have suffered absolutely no overheating effects, even in summertime.

This does not at all change the fact that this is potentially dangerous advice. How do these people know that they are not one serious cross wind from a serious scare? Did they know this before taping their car up? I'd wager not. Full facts are required before someone attempts this. Any affects on high speed stability will only be apparent at those high speeds, and any potential aerodynamic stalls may be impossible to stop when this occurs - total loss of air to a radiator (as I have seen in the past) can cause the car to overheat extremely quickly. In one case it caused total engine failure and the car crashed because of it. This is not something to take lightly, no matter how many people have got away with low tech mods for fuel consumption.

I take offense at your 'misguided' remark

Your 'fit a flat floor' advice is about as misguided a piece of advice as I have seen. I stand by my statement. Aero modifications should NOT be attempted without proper knowledge and investigation. It is not a suggestion that should be tossed into an internet forum blithely without qualification. That is what I am doing.
posted by Brockles at 6:28 PM on May 1, 2008


Full facts are required before someone attempts this.

Apparently not, because lots of people are doing it with complete safety. I would say that I'm sorry that bothers you, except that I'm not. One hardly needs to become an expert in something before attempting experimentation.

Your closed-mindedness and condescending attitude has worn thin. I'm done here.
posted by eratus at 4:25 AM on May 2, 2008


Your closed-mindedness and condescending attitude has worn thin.

Closed minded? I develop and race cars for a living. A closed minded person wouldn't survive in my business. There is nothing closed minded about saying "there is more to consider about the holes in the front of your car than just the radiator". In fact, you assuming that this is all they are for shows a lack of understanding of the other aspects of airflow over and through a car, and it is closed minded of you to refuse to allow yourself to be corrected by someone pointing out the fundamentally linked larger implications.

If you find someone with more information than you correcting you 'condescending', then I'm sorry for you. I'd assume someone who spends their time modifying cars would want to do as good a job as possible, using all the information.
posted by Brockles at 11:06 AM on May 2, 2008


Hmm.... the condescension is quite thick here...

ASIDES:

My present driving habits get me an average of over 32 mpg (combined city/hwy) in a car that was rated at 27 mpg hwy.

Fascia mods: my car has a number of "air intake" features on the front of it, some functional, some not.

Wing: if the (factory) rear wing makes my car too unstable at highway speeds, I'll simply put it back on. Whereas I do not expect removing the wing *alone* to make a significant difference, hopefully the cumulative effect could be appreciative. As far as reducing drag - no, a rear wing is not going to reduce drag; you can smooth air flow at the rear with the design of the back of the car, but interrupting airflow creates drag - there's no way around that. In racing it's a trade off for downforce, but I'm not racing.

The underside of my particular model car already has plastic panels that cover most of the underside, BTW...
posted by raikkohamilonso at 8:05 AM on May 4, 2008


My present driving habits get me an average of over 32 mpg (combined city/hwy) in a car that was rated at 27 mpg hwy.

That's pretty good as a percentage increase. There may come a point that effort/cost for reward is not worth it, though. There's only so much you can do with the vehicle you started with.

Wing: if the (factory) rear wing makes my car too unstable at highway speeds, I'll simply put it back on.

Assuming you don't have an accident finding out the stability is affected. That's a risk you need to understand before you try it, that's all. It should not be dismissed as an inconsiderable risk by people that don't understand it, which was my concern. If it was placed on for stability purposes (as in the Audi TT - we still don't know what car you drive) then it is dangerous to remove it. A company doesn't produce a part for global recall and retro-fitment for fun, as I'm sure you realise.

As far as reducing drag - no, a rear wing is not going to reduce drag

This is why I specified wing or spoiler. A wing (airflow on both top and bottom) will produce drag while producing downforce and stability. A spoiler (air flow on only the top - jutting up from the rear hatch/boot lid for instance) can solely reduce drag. As such, removal of a spoiler is potentially ill advised - especially if it is factory fitted. There's not much doubt it was wind tunnel tested to produce a lower coefficient of drag and extra stability.

interrupting airflow creates drag - there's no way around that.

This is not at all true for all cases. Aerodynamics is much more complicated than that suggestion implies, which is why I have been attempting to discourage bodged, ill advised 'solutions' based on incomplete awareness of aero. Hence why streamlining is a blanket concept that has been abandoned. Spoilers produce turbulent flow around a component (or a car) that allows the normal (laminar) flow of air to recover from the disturbance of the car sooner (turbulent flow has more inherent drag, but reverts to normal faster then a purely laminar disturbance, reducing overall drag). Basically, if you are going to disturb the air, then you also need to 'trip it up' to get it to recover from the disturbance faster. A gentle return for the air can cause more drag than a faster, more turbulent, return.
posted by Brockles at 10:42 AM on May 4, 2008


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