How do you make sure your car is getting the most mileage possible?

May 29, 2005 12:55 AM

I'd really like to see what tricks of the trade you guys have to make sure your car is getting the most mileage it possibly can.

I'm not interested in "buy a more fuel efficient car"- I'm already doing so. I want to know what driving habits I can pick up to make whatever I'm driving even more efficient.

For example, I've learned that if I accelerate slowly and stay below 2k RPMs (usually about 60mph on the freeway), I get almost 4 additional miles per gallon on my car then I do normally.

Is there anything else you guys would suggest?

posted by BuddhaInABucket to travel & transportation (38 comments total)

1) If your car's intake system is unreasonably restrictive, consider some simple modifications to make it less so. For example, on a 2003 Hyundai Elantra, one can remove the resonator box from the air intake. It's there to keep the engine quiet but removing it lets the engine breathe a little freer. If you don't want to get so involved, try a K&N air filter, they have them in sizes that fit most cars. The K&N was good for about an extra MPG highway on the Elantra. As an added bonus, it actually captures more dirt than regular filters, and you only have to change it once every 50,000 miles, so in terms of cost it comes out about the same as regular filters.

2) Ditto for the exhaust -- remove restrictions. This can mean anything from a high-flow muffler to a complete custom exhaust pipes bent so that the full pipe diameter is maintained through the bends (usually, it isn't).

3) Good spark plugs, if your car has crappy OEM plugs. Platinum or iridium plugs. I put Bosch Platinum +4 plugs in the Elantra and got another 1 MPG highway.

4) Try an oil change with synthetic oil. You can go twice as long between oil changes so it doesn't actually cost any more (though double-check to make sure this won't affect your car's warranty). I didn't notice any difference in fuel efficiency with synthetic oils but some people do.

Basically, almost anything simple that you could do to improve your car's performance will also improve gas mileage. Increased efficiency = more power from a given amount of fuel = less fuel for the same amount of power; they are synonymous. If you get into the habit of driving with the least amount of power you need, you will reap the benefit from these kinds of tweaks. Nitrous injection system, not so much.

Such tweaks pay for themselves pretty quickly these days. 2 extra MPG on a 33 MPG car is 6%. When gas is at $2.50 that's like saving 15 cents a gallon or two bucks a tankful. Of course you probably don't do 100% highway driving, nobody does, but considering I paid only about $20 for the new set of plugs (the air filter comes out even considering I bought one for $35 and still haven't replaced it 35,000 miles later) I'm pretty sure I'd be ahead of the curve now even if gas cost $1 a gallon.
posted by kindall at 1:22 AM on May 29, 2005

Round up the usual suspects: tire inflation, tune-up, no unnecessary cargo. You've found the benefits of light acceleration, but there's another driving trick: avoid unnecessary braking. If you leave enough space between your car and the one ahead, you often won't have to slow at all when they brake, since they'll speed up again after. Your goal is to keep the engine running at a constant speed; it's accelerating that uses the most fuel.

Bear in mind that practicing those last two tricks to extremes will make you very unpopular in heavy traffic.
posted by Kirth Gerson at 1:59 AM on May 29, 2005

I was taught in driving school that one should accelerate relatively swiftly to whatever speed you want to drive then maintain that speed with a high gear and low rpm. It often makes sense to skip a gear when shifting. For example: Accelerate to 50mph on third gear then shift directly to fifth gear. When slowing down, don't use the brakes, downshift and engine brake, new car engines don't use any fuel when you are engine braking. Ideally you want to be driving at the same speed all the time in as high a gear as possible.

I see you are not living in a cold climate, but anyone who does, should get a block heater. A cold engine uses more fuel and pollutes much more.
posted by lazy-ville at 3:00 AM on May 29, 2005

Note that the kind of modifications that kindall suggests can void your warranty and may cause your car to fail a smog inspection, important considerations if you live in a state like CA or NY.

Other than that, try to drive as smoothly as possible - no sudden stops or starts.

Turning off the A/C can help in smaller cars, in large cars it's often not a factor.
posted by madajb at 3:43 AM on May 29, 2005

Accelerating "swiftly" is exactly the event that uses the most fuel. New car engines do, of course, use fuel when engine braking. Downshifting causes the engine to speed up, which uses more fuel. Basically, more throttle uses more fuel, and higher RPM uses more fuel. A light foot on the throttle will always give better mileage.

If you've got a manual transmission, you can try skipping gears when speeding up, but if you have to go to a higher engine speed in say, second, so you don't lug in fourth, you won't gain.

Unless you live in a place that sees subzero (F) temperatures, you don't need a block heater. It just transfers the fuel cost to your electric bill, and the pollution to wherever the power plant is.
posted by Kirth Gerson at 4:07 AM on May 29, 2005

Accelerating "swiftly" is exactly the event that uses the most fuel. New car engines do, of course, use fuel when engine braking. Downshifting causes the engine to speed up, which uses more fuel. Basically, more throttle uses more fuel, and higher RPM uses more fuel. A light foot on the throttle will always give better mileage.

The engine control unit in new cars won't inject fuel into the engine when engine braking. I'm not talking about drag race accelerations here, simply that you accelerate to the wanted speed so you can then drive at a lower rpm and with less throttle than when constantly accelerating.

And I disagree about the block heater, if only because you are not only reducing fuel consumption but also significantly reducing wear and tear on your engine.
posted by lazy-ville at 4:49 AM on May 29, 2005

I know it's obvious, but drive less! Plan on what you need, and combine your trips rather than making several. Pick up food, buy gas, stop at the hardware store, all on the way home from work.

Unless you're out in the wilderness, it wouldn't hurt to walk or bike a bit either.
posted by Marky at 5:07 AM on May 29, 2005

I walk everywhere, which saves plenty of fuel. My partner (who now takes advantage of my monthly fuel allowance) saves a great amount of fuel by negotiating starting and finishing times at work. She gets on the way to work before rush hour, and leaves before rush hour.

This takes an hour long slog through slow traffic and makes it a 20-30 minute cruise to work, saving a LOT of fuel (almost a tank per week).

Keep your car serviced, keep extra weight out of it (I had a boot full of heavy stuff I used to lug everywhere), make sure your tires are properly inflated.
posted by tomble at 5:22 AM on May 29, 2005

How does one measure gas mileage accurately enough to know that such and such a change has caused an increase?

(My car doesn't have some kind of automatic computery thing to tell me. My parents write down numbers from the odometer when they get their tank filled, but that can't be very accurate unless you're sure the tank is totally empty, right?)
posted by bcwinters at 6:26 AM on May 29, 2005

Note that the kind of modifications that kindall suggests can void your warranty and may cause your car to fail a smog inspection, important considerations if you live in a state like CA or NY.

Not necessarily on either point. Putting better air filters in your car and freeing up your exhaust from the catalytic converter to the back are perfectly legal in all states and I haven't heard of warranty violations from either activity. "cat back" exhaust systems are quite popular everywhere as a result.
posted by mathowie at 6:28 AM on May 29, 2005

bcwinters, you just zero out your tripometer after each fuel up, and note how many gallons you added to the tank. If you don't have a tripometer, use the odometer reading.

Last fuel up, car had 6253 miles on it. This fill up it had 6496 on it, and I used 9.65 gallons of gas to fill.

(6496-6253) miles/9.65 gallons = 25.2 miles/gallon

You do this calculation each time you fuel up and if you're anal about it, you keep track and plot in an excel graph. Personally, I use the calculator function on my cell phone each time I fill up and just note it mentally. My honda element is rated for 21city/24highway and I consistently get 24-25mpg in mixed conditions.
posted by mathowie at 6:35 AM on May 29, 2005

Not trying to derail: Bcwinters, writing down numbers from the odometer is accurate as long as you're sure the tank is totally full. Imagine filling up the tank and setting the trip odometer to zero. The next time you fill up the tank, you can easily determine your mileage by comparing the amount of gas you just purchased with the distance you've driven since you last filled up the tank.

Back to maximum mileage: I don't think anyone has mentioned wind resistance. Keep the windows up. And, on the highway, draft behind other vehicles. Come to think of it, I don't think anyone has mentioned driving slowly, at least on the highway. In most cases, 55 mph consumes a lot less gas than 65 mph, and a whole lot less than 75 mph.

On preview: Matt beat me to the mileage thing.
posted by box at 6:38 AM on May 29, 2005

I see you are not living in a cold climate, but anyone who does, should get a block heater. A cold engine uses more fuel and pollutes much more.

And don't "warm up" the engine by letting it idle for minutes at a time. A cold, idling engine burns fuel extremely inefficiently, depositing a lot of acidic byproducts into the oil, which then go on to harm rings &c. Run it for a minute to let the oil get to all corners of the engine, and go.

on the highway, draft behind other vehicles.

Keep off my ass end. At highway speed, there sure as hell better be adequate braking distance between my bumper and yours.

I don't think anyone has mentioned driving slowly, at least on the highway.

That's because driving significantly slower than the surrounding traffic is damned dangerous, possibly illegal, and almost always stupid.
posted by five fresh fish at 7:17 AM on May 29, 2005

i've used mathowie's method and there is a lot of noise in the data due to the fact that it's hard to judge when you're tank is full when filling up. some gas pumps turn off sooner than others. but if you take enough data eventually you will see trends in the data, and you can notice things like getting better mileage when you take longer trips vs. a week of short trips. if your mileage starts to drop unexpectedly, you might realize that it's time to have your car looked at.
posted by jacobsee at 7:27 AM on May 29, 2005

RE: downshifting to slow down, I (and Click & Clack) recommend against it. It's fun, but ultimately it's far cheaper to replace worn brakes than it is to replace a transmission.

Back on the topic of calculating mileage, has anyone gauged the accuracy of onboard MPG readouts?
posted by schoolgirl report at 8:56 AM on May 29, 2005

Slip into neutral on long stretches when there's no one on your tail, especially around speed drops. Coming into town the speed goes from 55 to 25 over maybe a quarter of a mile. If there's no one behind me, I just slip it into neutral at the beginning and am going the right speed by the time I hit the 25 sign. Cars coast very, very well in neutral.

Note that if you're in traffic, being in neutral isn't necessarily a good idea just because it takes that much longer to shift back if you need to suddenly accelerate, and for stopping you don't have the engine behind you.
posted by devilsbrigade at 9:20 AM on May 29, 2005

has anyone gauged the accuracy of onboard MPG readouts?

I have an onboard MPG display on my Malibu Maxx (hold the guffaws, this is one of the best US-made models in terms of price/value ratio, IMHO). At each fill-up I reset the display and do a manual calculation of mileage. Consistently the display is 5-10% higher than the calculated amount. No reason that I can find. The same was true on my 1989 Taurus which had a similar display.
posted by beagle at 9:24 AM on May 29, 2005

Coasting is not good for automatic transmissions, especially not with AWD.

Block heaters do not save energy. Let's say you've got one on a thermostat set to go on when air temperature drops to 32 F. So you plug it in when you get home, and an hour later, it gets cold out. The heater goes on. 12 hours later, you come out, get in your warmed-engine car, and drive off. You've kept your engine warm all night long, unless you've also got the heater on a timer. Do that for a week, and you've expended far more energy than you would have by getting in a cold car and driving it for the five minutes per trip it takes to warm it up (unless you do a lot of swift acceleration and downshift-engine-braking while the engine's cold, of course).

Your mileage will vary.
posted by Kirth Gerson at 9:37 AM on May 29, 2005

Hey Kirth - I'm sure you don't mean it that way, but in a lot of places you need to use a block heater in the winter to ensure that your car will actually start. After a good solid night at even -10 or -15, some cars are very difficult to turn over. In even colder weather, some cars won't even turn over.

Be like smart people and put your block heaters on a timer. Set it for two hours before you need to leave for work, three if it's exceptionally cold out or if your car isn't sheltered.
posted by nathan_teske at 9:54 AM on May 29, 2005

RE: downshifting to slow down, I (and Click & Clack) recommend against it. It's fun, but ultimately it's far cheaper to replace worn brakes than it is to replace a transmission.

With over 250 000km on my manual NX 1600, over twelve years of aggressively using downshifting, the transmission is in perfectly fine condition. In my opinion and experience, the "replace a transmission" worry is bullshit.
posted by five fresh fish at 10:52 AM on May 29, 2005

The Ecodrive project has some tips, most of them already included here.

This is what Ecodrive e to say about engine braking:
"Petrol and diesel cars manufactured from 1990 onwards, are generally equipped with fuel injection combined with an electronic function that cuts off the engine's fuel supply under engine braking..."

Some information and research about pre-heating your engine by Motiva.(First link is a PDF in Finnish.)They recommend using a block heater for 30 minutes when the temperature is below 5 °C (40 °F) and for up-to 2 hours when temperatures fall to 0 °F. Of course keeping your engine block warm for the whole night will use more energy than running a cold engine. But warming your engine before running it is good for your wallet (at least with finnish gas and electricity prices) and very good for the environment. A cold engine is horribly inefficient and polluting.
posted by lazy-ville at 10:58 AM on May 29, 2005

In most cases, 55 mph consumes a lot less gas than 65 mph, and a whole lot less than 75 mph

Depends on the car, I think. My old Probe and my current Prelude tend to do best between 65 and 70, it seems. Hard to say though since trips doing 60 will be off interstates and have some small number of starts and stops in the odd town here or there instead of being 'Get on interstate, drive for four or five hours, get off interstate for whizzular purposes."

Thing about doing 55 instead of 75 is 75 is almost half again as fast, so you save an hour on a three-hour 55mph run. Ya gotta figure the time value of those lost hours in somewhere.
posted by ROU_Xenophobe at 11:05 AM on May 29, 2005

ROU_Xenophobe: on really long trips the time value adds up, but for average to medium-long trips, the difference is negligible. When you're going on a 70 mile trip, the difference between 60 and 70 MPH is only ten minutes- besides getting better fuel economy (my car does best at 60mph), you're also a LOT more likely to survive an accident.
I am in the process of trying to outgrow the habit of leaving late and rushing to wherever I need to be (long-term plan to lower my insurance rates), and that's how I noticed the extra fuel savings.

Marky, Tomble: I live in the city right now and I walk as often as possible, but in a few weeks I'm moving back to the suburbs... If you live in southern california you know that "walk more!" is pretty much impossible.
posted by BuddhaInABucket at 11:25 AM on May 29, 2005

Another good one I forgot originally: tires with low rolling resistance. Michelin has the Energy MXV4 which can save you a bit. My car actually came with a set of these. If yours didn't, check 'em out. If your car is fairly new, you might actually be able to get a discount on a new set of the Michelins by trading in the tires that are already on it (check with Discount Tire or America's Tire, whichever one you have locally, they do this).
posted by kindall at 1:04 PM on May 29, 2005

K&N air filters were mentioned above as needing replacement only every 50,000 miles. You don't actually have to replace them. You can wash and re-oil the filter basically indefinitely.

I use them in my two vehicles. You should use one in your car.
posted by 6550 at 1:51 PM on May 29, 2005

mathowie writes "Not necessarily on either point. Putting better air filters in your car and freeing up your exhaust from the catalytic converter to the back are perfectly legal in all states and I haven't heard of warranty violations from either activity. 'cat back' exhaust systems are quite popular everywhere as a result."

No, not necessarily on either point, that's why I said "can" and "may".
But modifying the intake(by removing the resonator) would most likely be a CARB violation and probably would void a warranty as well if someone takes the time to notice.
Not a big deal for most people, but something to take into account if you are due for a smog or dealer service anytime soon.

If he was talking cat-back, then yes, you are right, no problem there, but I got the impression he was talking about a full exhaust, in which case if you are heading for a smog test, you might want to think twice about it.
posted by madajb at 3:51 PM on May 29, 2005

nathan_teske, yes, I know about cold oil. That's why I said, "Unless you live in a place that sees subzero (F) temperatures, you don't need a block heater." If you do live in such a place, you may very well need a heater. And a timer. And a gasoline, not a diesel engine.
posted by Kirth Gerson at 6:20 PM on May 29, 2005

"Slip into neutral on long stretches when there's no one on your tail, especially around speed drops. Coming into town the speed goes from 55 to 25 over maybe a quarter of a mile. If there's no one behind me, I just slip it into neutral at the beginning and am going the right speed by the time I hit the 25 sign. Cars coast very, very well in neutral.

Note that if you're in traffic, being in neutral isn't necessarily a good idea just because it takes that much longer to shift back if you need to suddenly accelerate, and for stopping you don't have the engine behind you."

...which is why it's illegal to coast in neutral in many (if not all) states.

just don't do it. saving a buck on gas isn't worth not being able to maneuver out of an emergency situation.
posted by Ziggy Zaga at 6:29 PM on May 29, 2005

nathan_teske, yes, I know about cold oil. That's why I said, "Unless you live in a place that sees subzero (F) temperatures, you don't need a block heater." If you do live in such a place, you may very well need a heater. And a timer. And a gasoline, not a diesel engine.

Do you just make this stuff up as you go? Why would one need a diesel rather than a gasoline engine in a colder climate?
posted by lazy-ville at 7:02 PM on May 29, 2005

Do you just make this stuff up as you go? Why would one need a diesel rather than a gasoline engine in a colder climate?

Do you make up the stuff you think I wrote, without bothering to read it? I said "And a gasoline, not a diesel engine." Which is exactly the opposite of what you said I wrote.
posted by Kirth Gerson at 7:07 PM on May 29, 2005

[modifying the intake] probably would void a warranty as well if someone takes the time to notice

Under U.S. law, at least, the manufacturer would have to prove that that the modification caused the problem being claimed under warranty; they can't just blanket void your warranty. The odds of a less restrictive air intake actually causing any problems are so low they're not even worth worrying about, it's not like it's a turbocharger or anything.
posted by kindall at 7:11 PM on May 29, 2005

Oops, sorry. I said the very opposite of what I meant to. Let me rephrase that: Why would one need a gasoline rather than a diesel engine in a colder climate?
posted by lazy-ville at 7:25 PM on May 29, 2005

Diesel fuel doesn't flow well when it's cold (it's thicker than gasoline). You do need a block heater for a diesel engine in a place that has winter, and you'd also be prudent to add a thinning agent to the fuel (unless the diesel vendors do that already now). My experience with diesel engines is way in the past; back then, it was far short of being worth the effort to use diesel.
posted by Kirth Gerson at 7:37 PM on May 29, 2005

Winter-grade diesel in here won't start to thicken until it reaches -29 °C (-20 °F). Obviously, using summer-grade diesel during the winter would be problematic. Any relatively new diesel-car worth buying anyways will also start in the winter. Eg. my parent's have a '99 Renault Scenic with a 1.9 liter turbodiesel engine and it starts without any trouble in 0 °F weather. (French carmakers, mindful their cars' old reputations as poor winter-performers tend to "winterize" cars sold in the north with better batteries, starters and glowplugs.)
posted by lazy-ville at 8:07 PM on May 29, 2005

Re: better mileage at higher speed - don't believe it. Think about it this way, if you are going 55, do you push on the gas pedal to go 65 or do you let up on it?
If you have an older car there are several interesting things you can do. Run low oil levels in the manual trans and rear so that the gears just barely dip in the oil. Change to a fan with less blades (or no fan at all if you never sit idling). I remember my father talking about people modifying cars for mileage during the great depression. They would block off the intake ports on some cylinders and replace the sparkplugs with breathers, turning an 8 cylinder into a 4 cylinder.
posted by 445supermag at 8:59 PM on May 29, 2005

?Diesel has problems in the winter? Not that I noticed back when our family has a diesel rabbit in the north half of Canada back in the 1970s. Indeed, I remember that it was able to do exceptionally well on icy hills, because it had such high torque at low RPM. It'd inch it way up at speeds that would stall a gasoline engine.

better mileage at higher speed - don't believe it. Think about it this way, if you are going 55, do you push on the gas pedal to go 65 or do you let up on it?

Heck, I've noticed that if I reduce speed from 10mph to 0mph, I don't use any gas at all!
posted by five fresh fish at 10:36 AM on May 30, 2005

A neat trick for slow acceleration is to drive without shoes on. Your feet will be more sensitive to the pressure on the accelerator and you'll automatically drive with a lighter touch.
posted by rocket88 at 4:01 PM on May 30, 2005

K&N air filters were mentioned above as needing replacement only every 50,000 miles. You don't actually have to replace them. You can wash and re-oil the filter basically indefinitely.

Yeah, but the re-oiling can be tricky. I've heard too many stories about people fouling their Mass Air Flow sensor by using re-oiled K&Ns. Getting that fixed will NOT be covered by warranty and it will more than blow any savings you think you're getting by re-oiling. My advice is to just get a new one; it doesn't cost any more than the paper filters over the same amount of driving.
posted by kindall at 9:00 PM on June 10, 2005