How might I squeeze a little extra mpg out of my old truck?
November 19, 2013 5:41 PM   Subscribe

I've become mildly obsessed with trying to improve the fuel efficiency of my old truck. I've already done all the basic things and I realize that I'm fighting physics at this point, but what else might I consider if I wanted to make a bit of a hobby out of it?

So I have an old truck (a 1994 4Runner, as you may have heard) and I pretty much love it. It's inexpensive and reliable (just got back from a 1200 mile road trip in which the odometer passed 200,000 miles without a hiccup) and it carries lots of stuff when I need it to. It's nearly a perfect vehicle for me.

The only problem is the inevitable poor gas mileage. I've got the RWD, 3.0L V6 version of this truck and I get about 13.5 mpg in town and 16.5 mpg on the highway. I realize that at the end of the day you can't beat physics and that there's not a heck of a lot I can do about this, but as a sort of hobby-slash-intellectual-exercise I've been trying to do what I can to make the best of a bad situation.

I'm already doing all the basics, I think. I drive as slow as I can without disrupting traffic, and I accelerate gently whenever possible. When faced with a hill I'll let the speed come down a bit on the uphill climb (again, if I can do so without disrupting traffic) and then let it come back up on the downhill coast. I take long coasts up to stoplights whenever I can. I keep the tires at the recommended pressure and I change the oil and transmission fluid regularly. I don't carry a lot of extra cargo (a few tools, yes but hardly every last thing that was suggested in that AskMe). I've removed the wind deflectors and spoiler that were on the truck when I bought it, and I've taken the crossbars off the roof rack and keep them stowed in the back for use as necessary.

So that's all of the simple/free stuff, I think. I'm now considering stepping up my game a bit and trying some minor modifications – again, approaching it more as a hobby and realizing that I'm hitting diminishing returns pretty hard here. I've read around about things that one can do, but I don't really trust the hypermiling forums and such to give advice that is even moderately practical. So, HiveMind, I come to you.

What are some things that I could do, if I wanted, that would be at least theoretically worthwhile? I'm looking for practical ideas, for a suitably limited definition of "practical". To be worth considering, a modification (whether behavioral, mechanical, or aerodynamic) would have to satisfy the following criteria:
  • Must provide real benefits in real-world driving. It's OK if the benefits are very small.
  • Must be free, very inexpensive (less than $25), or pay for itself within about 20,000 miles.
  • Must not impact the reliability or safety of the vehicle. Funny-looking is OK.
  • Must not have a major impact on practicality. Minor impacts will be considered case by case.
I'm interested in any suggestions you might have. Once again, I am approaching this as a nominally-practical hobby rather than as a way to have a serious financial or environmental impact. I'm aware that the best thing I can do in real life (assuming I stick with this truck, which I am doing) is to simply drive less and I'm definitely working that end of the problem too.

Semi-aside: one specific avenue I'm curious about is fuel-monitoring equipment. My truck is too old for OBDII so a ScanGauge isn't an option, but something like an MPGuino or even just an old-school vacuum gauge is a possibility. Does anyone have any experience or recommendations in that area? This is something that I'd be interested in trying just for the sheer geekiness of it, assuming it's not just a giant pain in the ass.

Thanks as always for your advice, it's very much appreciated.
posted by Scientist to Travel & Transportation (28 answers total) 3 users marked this as a favorite
 
Weight reduction and ghetto aero are probably where it's at. Look at this old MotoIQ series for the kind of stuff you could do. Other possible inspiration: the kind of modifications Miata weight-weenies do. The last possibility I see is doing stuff that will improve the functionality of your truck while also reducing its fuel usage, like lighter wheels, lighter suspension components, lighter brakes. But the added functionality has to be worth it besides the mileage improvement.
posted by Monday, stony Monday at 6:05 PM on November 19, 2013


Two semi-easy fixes.

Tires.
- Make sure you have decent tires on decent rims aligned correctly. Junky tires on an old truck are gonna really kill some easy mpg. Even if they're well pressurized. If that vehicle has original wheels/rims, l'd consider replacing them. I'd for sure get an alignment.

Air Filter.
- Well, it's more than that, it's intake as a whole. If you are in humid conditions, a dirty air filter could kill even more mpg. And cleaning an air filter is as easy as finding someone with some compressed air.

YMMV.
posted by Sphinx at 6:24 PM on November 19, 2013


The first thing I did when I got my old jeep (before I switched the license plates, before I removed the stupid stickers, etc.) was run seafoam through it.
posted by oceanjesse at 6:26 PM on November 19, 2013 [1 favorite]


Engine efficiency. Either through improved intake or exhaust. Basically you're not going to get more power (as that's not what you're after) but you can remove efficiency issues to produce the same power at lower rpm's (which will show a fuel reduction to some very small degree).

A flat cover for your truck bed (you don't mention that) that is as rigid as possible (flapping canvas ones cause drag).

Worth considering if you don't go offroad - a flat underfloor. Use very thin ali or steel sheet (which can be cheap when very think) and just rivet a sheet underneath the car that doesn't interfere with suspension or steering movement and get rid of all those pockets (like the truck bed on top) that holds air and causes drag. You can do from the firewall back if you want to make sure you don't have to experiment with air out-take from the engine and affect cooling.
posted by Brockles at 6:28 PM on November 19, 2013 [1 favorite]


I've gotten 13-16 mpg out of a 3/4 ton suburban with a 350 v-8, so you should be able to do better. Is it 4x4? If so you could switch to manual locking hubs if it doesn't have them. Another thing that will kill mileage is a clogged catalytic convertor. All the sensors working (especially O2 and MAS)?
posted by 445supermag at 6:53 PM on November 19, 2013


Do you have rear seats? Do you need to have rear seats? They could be removed to reduce weight. Clean and wax it - tiny efficiency, but also protects paint.
posted by theora55 at 7:13 PM on November 19, 2013


Over-inflate your tyres a bit (probably not in winter, depending on your climate, to respect your safety caveat).
posted by pompomtom at 7:37 PM on November 19, 2013


Exchange the distributer for a new or rebuilt unit, then replace the wires and plugs, even a slightly worn distributer ruins performance and milage . agreeing with 445supermag also.
posted by hortense at 7:54 PM on November 19, 2013


Mostly you are trying to fight physics. Even a brand new 2wd 4Runner with the small engine gets barely better mileage according to the EPA, and that's a vehicle with another twenty years of engineering.

I'm not suggesting you really do any of this, but you would get gains by: lowering the car and installing air dams. Switching to skinny, low-friction tires. Increasing the final drive, either by changing the gears in the differential or switching to larger tires. Have you seen tractor trailers these days? They don't just have side skirts and rounded things on top of the cabs, but more and more they have tapered deflectors on the back of the trailer -- theoretically you could do the same on the Toyota to give yourself more of a teardrop shape.

But at the and of the day, your gains are going to be pretty paltry because it's an inefficient engine pushing something the shape of a brick. Take care of the cheap easy stuff (like the tune up, alignment, and tire inflation), and see where that gets you before taking on the high cost options.
posted by Dip Flash at 9:23 PM on November 19, 2013


Car manufacturers seem to go to a lot of trouble to reduce weight as a way of bringing down consumption. There would be plenty of things you could do there no doubt.

If it's not fuel injected, then making sure your carburettor is in good condition helps. Worn parts can make a big difference. If it is fuel injected, then making sure that's working as designed might help too.

From what I understand, recommended tyre pressures are for comfort rather than anything else. I usually run mine 20-40% over the recommended. They last longer, have less drag and, in my case, still grip extremely well in the wet. You need to experiment with that though and not do anything too rash. And be prepared for a rougher ride.
posted by mewsic at 9:36 PM on November 19, 2013


My Yaris drives with only 1/3 of the backseat in place and no spare. I don't know if your ride has bucket seats but you could strip the spare out if needed.

Basically you seem to have a good grasp at the places where gains are to be realized, mainly:

1) Driver behavior, which you seem to be covering, although reading up on more advanced tactics (doubly so if you have a standard) on hypermile forums might be helpful. Buy a SCANGuage! Mine has helped train me and improve my behavior so much, plus it's a great diagnostic tool if your instrument cluster is only comprised of idiot lights like mine.

2) Routing choices. Don't drive when it's high traffic. Pick routes that put you at the mileage sweet spot for your vehicle for as long as possible (the scanguage will help with this tremendously!), and watching the road ahead of you for traffic pattern / red-light concerns that you can predict are all good things to keep in mind.

3) Mechanical options. Weight and drag reductions are going to be the low hanging fruit here. Also, as others have mentioned, don't skip tire/wheel alignment and maintenance, higher pressures being better. Some people go to great lengths to minimize drag but I doubt you're looking to go that far, but still perhaps you could learn something you didn't know. Mechanical tricks like picking the right weight of oil and using synthetic transmission fluids are helpful but are also a bit advanced, I think, for your query.

4) Minimizing overall use. This sort of falls under 1) but if you save a trip back to town you've just cut your environmental impact in half. Don't forget your lunch at home and have to turn around and go back for it. Don't idle in your driveway to "warm up" your car/truck, that's been unnecessary since our parents/grandparents days: Get in, start the car, and by the time you're buckled up and comfy the car's ready for you to drive it like a sensible human being.

5) Throwing comfort out the window. Running the AC makes the engine work harder and burns more fuel. So you go without. But that means you need to roll down the windows, which increases drag (but only perceptibly so at higher speeds, remember). So you roll them up. But it's HOT! Window tints and dressing properly will help with this. Also drink a glass of ice water before you leave. The inverse is true of [not] needing the heater. Drink some hot coffee and skip the Bermuda shorts in January. The defroster is trickier, some, mostly newer, vehicles assume that if you set the defroster on you also want the AC to run and remove humidity from the air. While helpful it may not be completely necessary, so there are ways to circumvent that and just get air-vent instead of vent+AC when you set the dial to defrost. Again, probably beyond the scope of what you want to do.

Uhh.... yea, that's all I got.... Get a ScanGuage. I've had a ScanGuageII since month 3 with the Yaris and love it.
posted by RolandOfEld at 10:03 PM on November 19, 2013


I forgot to mention you'll also save on brake pads! Making this a win-win-win-win-win. I mean, who doesn't want longer lasting brake pads. It's practically the american dream in two little ceramic plates.
posted by RolandOfEld at 10:07 PM on November 19, 2013


Definitely get the vacuum gauge, and read up on it. Under 30 dollars.

Another thing you can do is ditch your cold air input system. You want your intake air to be as warm and as freely available as possible. This reduces the amount of oxygen your engine can breath and hence generally reduces the amount of gasoline that you can burn, at the cost of acceleration and high end performance. Take your engine intake air from as close to the intake as you can, without some long duct bring in cool, dense air from outside of the engine bay. I am not suggesting that you remove your intake air filter. About 30 dollars.

When was the last time your oxygen sensor(s) were changed? Chances are good that it's been more than 60K miles (you're probably on the original one). I'm not familiar with the 4Runner, but if it has 2 oxygen sensors, the most important one for engine control is the one upstream of the catalytic converter. It has a direct effect on fuel economy. Over 30 dollars, but probably worth it.

Of course, change one thing at time, and see what the results are.
posted by the Real Dan at 11:01 PM on November 19, 2013


My car has an instant and average MPG readout, which I pay attention to, and some things I've noticed that are true of my vehicle - and which might be true of yours or might not be:

Getting up to 30, then maintaining 30 for a couple of blocks burns more than getting up a smidge higher than 30 and coasting in neutral for those blocks. Even when I use such a high gear that the engine is practically at idle anyway, taking it out of gear makes a difference.

Hills - my car is newer and the fuel injection is electronic. So if the hill is steep enough to coast down in a gear that will raise the RPM a decent amount past idle, the car stops using fuel altogether and runs everything off gravity-powered wheels. Due to another curiosity of my car (engine is oversized), I don't lose all that much extra fuel going up a hill (big engine is going to drink a lot regardless), but I save a lot coming down by having the car in a gear that compromises between low drag and getting the RPM high enough to shut off the fuel, and the result is higher average MPG over hilly trips than on flat trip. Your truck might be similar, or it might not.

Experiment with higher gears. I can drive up a hill with the engine idling by putting it in 6th gear and going at 30. I think this is a way of losing money in the long run (straining the drivetrain to save pennies on gas - probably not a money saver, but if higher MPG is a hobby...)


But the recurring theme in all of these is that I found them using the instant and average MPG meter, and they are likely peculiar to my car - different speeds and different gears would produce different tricks and sweet spots in different vehicles, so I think my real advice is that yes, looking into installing some kind of MPG meter is probably far and away the best action you can take to improve your MPG. As to how... no idea. Sorry.
posted by anonymisc at 1:03 AM on November 20, 2013 [1 favorite]


5) Throwing comfort out the window.

While I'd agree that turning down AC will give some benefit to reduced load on the engine (and hence fuel consumption) going quite so dramatic as this is likely to be a safety issue. Driver fatigue through discomfort is a very worthwhile consideration and if you go as far as to need a glass of iced water before you go to tolerate the interior of your car than it is a distraction and a safety issue. I'd go with windows down at low speeds and minimal AC with windows up at higher speeds.

The inverse is true of [not] needing the heater.

Not really, no. The car produces heat as a by product and has to reject that heat. It uses no extra fuel at all to create that heat. Rejecting it to the interior of the vehicle rather than directly to atmosphere is only a difference of a tiny current draw to the interior fan. It's negligible.

Another thing you can do is ditch your cold air input system. You want your intake air to be as warm and as freely available as possible.

This doesn't make any sense. Making your car inefficient will not make it use less fuel it will make it use more for a given driving style. Having cold air in the intake makes more power, but that allows you to accelerate to speed more easily and need less throttle angle to maintain that speed and keep your rpms down. You save more fuel if your car is able to produce the (say) 75 or so bhp required to maintain 50mph at 2000 rpm than if it is struggling to breathe properly and needs 2500 to produce 75bhp.

the result is higher average MPG over hilly trips than on flat trip.

Something else must be happening there. It doesn't scan from a physics perspective that raising a truck up and down (hence extra energy required) uses less fuel than to push a truck along an equal length flat road. Some other factor (like average speed) MUST be in play here if you see less fuel used going over hills. It's just not possible otherwise. I suspect the flat running is done at a higher average speed and hence aero drag is the presiding factor.

I can drive up a hill with the engine idling by putting it in 6th gear and going at 30.

This is power versus rpm (allowing for load). If your car is able to produce the power required to climb the hill at a lower rpm then it uses less fuel than it would in the higher gear as long as the engine is still in an efficient load band. If you get to the point of 'straining the drivetrain' then the engine is not running efficiently anyway and you are using more fuel than shifting down.
posted by Brockles at 5:13 AM on November 20, 2013 [1 favorite]


Yes, by all means don't passout while driving, your mileage will take a hit.

Also to second the whole "hilly routes aren't as bad as my brain said they should be", my results, again scangauge data based for good or ill, have been that if you don't have to stop and also don't have to brake hard on the downhill then the results can be strongly within my normal trip expectations.
posted by RolandOfEld at 6:12 AM on November 20, 2013


OK, so here's what I'm getting from this so far. Some responses, some questions.

Here are some things that I'm going to consider doing: some ghetto aero by gaffer taping over some of the panel seams and possibly retrofitting a flat belly pan. I will cautiously experiment with raising my tire pressure. If my intake filter is dirty I will replace it, and in any case next time I replace it I will fit a high-flow filter. I may also reroute the intake to give it a more direct path to the engine.

At the next service I will make sure to get the sparks, wires, and distributor checked and replaced if necessary. I will also have the alignment checked -- the truck tracks straight and true, but for all I know there's something more subtle going on that could use adjustment.

I'll eventually fit a vacuum gauge or an MPGuino (ScanGauge is not compatible with my truck, as I mentioned in the original question).

I'm curious and skeptical about the SeaFoam recommendation. From what I've heard in the past, fuel and engine treatments are basically the herbal supplements of car maintenance -- unregulated, unsupported by evidence, useless at best, and damaging at worst. I'm not reassured by the insistence by the author in the link that SeaFoam is "safe" because it is "100% petroleum-based". Candle wax (paraffin) is petroleum-based as well but that doesn't mean it belongs in my engine. At the same time gasoline contains lots of non-petroleum additives (ethanol, detergents, etc) which are perfectly fine for my engine. I have a suspicion for instance that the fact that the engine oil comes out black and thick after 100 miles of running with SeaFoam in it has more to do with the fact that SeaFoam makes a terrible engine oil than with it actually cleaning anything. If anybody has any solid information about this sort of thing then I'd be happy to hear it.

Brockles mentions the issue of efficient load bands for the engine. I've been trying to research this for my engine (a 3VZ-E) but I haven't been able to find anything that I can understand which seems reliable. Does anybody know what the most efficient bands are for this engine?
posted by Scientist at 9:14 AM on November 20, 2013


Does anybody know what the most efficient bands are for this engine?

A direct vacuum gauge will give you a good idea of that as they fairly accurately model load for your purposes. BMW's still use them and my old VW had one in the dash. The more vacuum in the intake the harder the engine is working to increase revs, essentially. That gives you a good indicator of load although it'd be harder to use it to judge mpg for accelerating in different gears unless you had a means of measuring rate of acceleration (possibly too much work).

I have a suspicion for instance that the fact that the engine oil comes out black and thick after 100 miles of running with SeaFoam in it has more to do with the fact that SeaFoam makes a terrible engine oil than with it actually cleaning anything

A lot of additives are snake oil, but if the oil gets markedly dirty in 100 miles, then it is certainly doing something. That has to be soot or build up in the engine and being as Seafoam goes in the fuel and not in the oil, it suggests that there is a transfer of dirt or build up into the oil (which would be where it should go). I've never used it personally, but removing carbon deposits will go a long way to improving internal engine efficiency.

Injector cleaner is also something that works, but the fuel additives you mention only work if they've always been used in the engine from new and the oil was always changed etc., etc. They can prevent build up but not remove existing build up necessarily.
posted by Brockles at 3:15 PM on November 20, 2013


>>Another thing you can do is ditch your cold air input system.

>This doesn't make any sense. Making your car inefficient will not make it use less fuel it
>will make it use more for a given driving style.

This does make sense. Reducing the average density of the air charge (warm bay air versus cooler exterior air) means that for a given engine power output you need a larger throttle angle. Larger throttle angle means lower pumping losses, and potentially better mileage.

It doesn't make the car inefficient. It makes it less powerful: At wide open throttle the amount of fuel that can be added to a warm air charge is less than what can be added to a cold air charge, but you do less than 10% of your driving at WOT. You'd probably barely notice the loss in speed, what with the cube law of power versus speed in air and all.
posted by the Real Dan at 5:17 PM on November 20, 2013


It doesn't make the car inefficient. It makes it less powerful

It makes the engine less efficient and that IS more inefficient for the car because the requirement is the total output (power required for moving a vehicle at a certain speed) and that requirement remains essentially constant. You can't just run the same speed profile at less power without using more throttle angle to compensate. Unless you just go slower, which less throttle angle in the first place (with the cold air intake) would provide (and would mean less throttle angle than with a hot air intake because the power is produced easier).

For another way to look at it, if your power production at a given rpm is reduced you will need more rpm (with hot air intake) or a greater throttle angle to produce the same power because you can't use less power to do the same job. So the net effect is a loss of efficiency and more fuel consumed (more rpm is more fuel used, more throttle angle is more fuel used).
posted by Brockles at 8:18 PM on November 20, 2013


for a given engine power output you need a larger throttle angle.

You're also perhaps forgetting that with a less efficient burn process you'd not just need a larger throttle angle, you'd also need higher rpm for the same power output, which screws the whole gains of pumping losses.
posted by Brockles at 8:21 PM on November 20, 2013


Brockles: Something else must be happening there. It doesn't scan from a physics perspective that raising a truck up and down (hence extra energy required) uses less fuel than to push a truck along an equal length flat road.

Nothing else needs to be going on there, it seems to makes sense from a physics perspective. Firstly, from a strict physics perspective, raising a truck up and down requires zero net energy - you recover all the energy you initially expend. (The same is true of the entire trip actually, in a frictionless vacuum, but my car lacks regenerative braking, and suffers from air friction). Secondly, internal combustion engines are crazy inefficient - and the engine in mine is (a lot) larger than necessary, such that the fuel burned just to keep the whole thing spinning is non-negligible - nearly as much as the fuel required to push the car along a flat, which in turn is a fair chunk of what is burned pushing the car up a hill. But zero fuel is consumed on the second half the distance, which is a huge savings, so the average can be lower than burning fuel to spin the engine for the entire duration along a flat.
In other words, the necessary factor to get higher MPG on hills is whether going up a hill requires less than double the fuel of driving flat. Using cruise control, driving flat I get ~30MPG, up a fairly steep (for freeway) hill I get ~18MPG.
posted by anonymisc at 11:21 PM on November 20, 2013


Or to put it another way, at freeway speeds, the most efficient power band for the engine is to be pushing twice the car for half the distance.
posted by anonymisc at 11:27 PM on November 20, 2013


you recover all the energy you initially expend.

There are losses in any conversion of energy. This is why it doesn't work. You never recover ALL the energy. Friction and air resistance are significant losses, so it is not zero net energy at all.

such that the fuel burned just to keep the whole thing spinning is non-negligible - nearly as much as the fuel required to push the car along a flat

I suspect you came to this conclusion because the engine is at idling speed? That doesn't scan at all because the fuel used to maintain idle is not purely linked to engine revolutions. 750rpm at zero throttle uses enormously less fuel than 750rpm with 1/4 or even 1/2 throttle. There is no way that your truck will push its way up that same hill without touching the throttle, therefore much more fuel is required than 'just to keep the engine turning'.
posted by Brockles at 5:15 AM on November 21, 2013


At 18mpg and 60mph you are burning 3.33 gallons per hour. At idle you will be burning a tiny, tiny fraction of that. Its unlikely your car will use more than 0.2-0.5 gallons per hour purely at idle.
posted by Brockles at 5:20 AM on November 21, 2013


This is why it doesn't work. You never recover ALL the energy. Friction and air resistance are significant losses, so it is not zero net energy at all.

And yet, it does work. I think you trying to intuit this from habitual rules of thumb rather than thinking it though - you bring up irrelevant things like air resistance, when the air resistance losses will be the same (or higher due to lower altitude) when driving flat; they are not a conversion loss. Gravitational potential energy being converted to kinetic energy is a far more efficient conversion than the massive losses an ICE generates simply by being in different parts of its power band.
I haven't personally matched gas-pump output against odometer against MPG meter, but I hang out on forums with enthusiasts of the same model who do, and they report the MPG meter was bang on. People with the same model have also reported better mileage on hills. They didn't expect it either. But it makes sense, once you consider the factors involved.
posted by anonymisc at 10:19 AM on November 21, 2013


Gravitational potential energy being converted to kinetic energy is a far more efficient conversion than the massive losses an ICE generates simply by being in different parts of its power band.

Right. Yet your massively inefficient ICE engine produced that gravitational potential energy (inefficiently) by using fuel in the first place (as well as pushing the truck along). So how does that factor? It may be efficient on the way down, but it sure as hell isn't on the way up. And there are losses all the way so it doesn't make sense from a physical point of view at all.

I'm not trying to intuit anything. It is harder to push a truck up a hill than it is to push it along the ground by a significant factor. If you are getting better fuel economy in hilly territory than flat land, then you're not allowing for some aspect of the equation. Engine rev band efficiency or ending at an overall lower altitude than you started or some other factor.
posted by Brockles at 10:39 AM on November 21, 2013


I'm not trying to intuit anything. It is harder to push a truck up a hill than it is to push it along the ground by a significant factor.

Exactly. A significant factor. And if that significant factor is 180% then you can come out ahead. You're already aware that the ICE efficiency is different at different loads. We have a situation where doing the same physics work more optimally falls in the over-powered engine's efficiency sweet-spot by pushing higher load for shorter time.
posted by anonymisc at 11:54 AM on November 21, 2013


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