Improving fuel economy
From Crankshaft Coalition Wiki
A basic list of techniques for improving fuel economy.
 Tires and wheels
- Keep tires inflated to proper pressure. Keep a small tire gauge in the glove box and check the pressure monthly, be sure to check all four. If you don't have an air compressor get one of those mobile emergency use units, or use the gas station's air pump.
- Exercise wisdom when choosing tires. Wide tires look nice, but present more rolling resistance to the ground. Use only the width you need. Also, take a look at the tire manufacturer's website and look at the actual tread width. The section width listed with the tire size is not indicative of the actual tread width that sees the road.
- Use that same wisdom when selecting those big 22" heavy wheels. Although wheels won't affect steady state MPG, the added rotating inertia can affect MPG if a lot of speed changes are used. It takes more right foot input to get big wheels up to speed.
- Another consideration is the tire's rubber compound. Touring and economy tires usually use harder compounds which sacrifice grip for longevity. As a very general rule, tread block size is often correlative. Smaller treadblocks like can be found on touring and utility tires often correspond with harder compounds. Harder compounds and smaller treadblocks can mean a little extra MPG help.
- Keep up with regular maintenance: clean air filter, properly gap and clean spark plugs.
- Maintain proper front-end or four wheel alignment if your car has IRS.
 Driving technique
- When stopped at a red light, do not shift to neutral as many sites will recommend. It is possible to verify with scanning software that the lower idle RPM reduces consumption, at least in fuel injected engines.
- Put a block of wood under your accelerator. No kidding. It really works, because of driving habits. Be mindful of how you apply your fuel, think of it in terms of energy, like the cyclist who is trying to keep his legs from aching by pedaling too hard. Coast down hills and get a bit of a run before upcoming hills. And whatever you do, DON'T keep working the accelerator on and off. Find a happy speed and hold it. Acceleration is the biggest energy burner.
- Pretend there is an egg between your foot and your accelerator pedal. It's an easy habit to develop and you can go about as fast as you like, it will just take longer to get up to speed. Unlike a block of wood, your pretend egg won't come loose, get trapped under the brake pedal, and cause you to plow headlong into a busload of nuns or something.
- Avoid the temptation to remove all the smog equipment in search of MPG. In many cases they help, especially EGR. The smog equipment started becoming mainstream in the early 70s, ushering in SAE standardized dynomometer testing and heavy EPA mandates on emissions. The low MPG associated with that era is mistakenly blamed on smog equipment, when in fact it should be blamed on abyssmally low compression and lack of today's technology. They were neutered muscle car motors.
- Consider diesel in your next vehicle purchase. Although diesel prices are much higher right now, the associated mileage you can typically achieve almost always more than offsets the cost. Other benefits are monster torque, resistance to the fuel being ignited in an accident and the ease of using environmentally-friendly biofuels. Plus, the resale value alone will usually more than offset any additional maintenance cost.
- For the more adventurous, fold in your mirrors and tape up the gaps around your grille and headlights. A respected diesel magazine was able to get almost 2 more MPG from a Cummins diesel Dodge with this modification.
- Consult with others who have the same model of car before hacking into the suspension, but, depending on the aerodynamics of your vehicle, most cars can gain a little MPG by finding the right stance.
- Don't assume a gear swap or overdrive will help. Best mileage occurs usually just below the torque peak, at or near peak VE. Making the RPMs too low can cause lugging, which means you'll be using more fuel trying to maintain speed. If you have a very high-rpm torque peak, then all bets are off, but chances are you're not worried about mileage anyway.
- Modern EFI cars are often tuned with a fuel map that keeps things a bit rich. This is for two reasons; first, to favor NO emissions at the expense of a little added HC emissions and secondly, to allow for a small buffer against detonation. Experienced reprogrammers (even Jet and Hypertech) use this information. By pulling a little fuel out of the curve, you can usually pick up 5 hp and sometimes just a little MPG.
- Resist the temptation to use a low-temp thermostat. True, it can make power in some circumstances, but at the expense of MPG and oil life. Drivers like their engines to run cool, but engines like to run hot for their health. Using a 195 stat can often times really make a noticeable difference in MPG, and your healthy oil will thank you. In a more modern engine, no or too low a temperature thermostat can have an opposite effect. The engine computer may believe the engine to be too cold and try to enrichen the fuel mixture accordingly. Using more fuel. VERY modern engines will simply recognize that the thermostat isn't working within expected parameters and work around it using less than optimal parameters. And illuminate the check engine light. Lose-lose.
- Avoid using exhaust crossover blockers. The hotter the intake air is, the better mileage it can get... of course at the expense of power. To some degree. Cold intake air means denser air. IE, more potential oxygen to be burned with the fuel to make power. But fuel atomizes a lot better when warmed, which is why those crossovers are there, to help prevent fuel "dropout" and puddling in cold weather. A compromise is to leave the crossovers open as stated but improve the path for cooler, fresh air to get into the engine. But expect very minimal measurable results either way. There are gains to be had in power and economy but they are best considered very small contributors to a larger picture and not usefully measured alone. (Unless you are in the advertising business and selling something.)
- On carbureted engines, sometimes people are not using the appropriate air/fuel mixture metering due to replacement carburetors. This results in greater fuel consumption than necessary. These same carburetors can be custom adjusted to a particular engine to operate at optimal performance, thus saving fuel.
- A four barrel carburetor can out perform a two or one barrel. This is because a four barrel in a street drive car runs on only the two primary venturis about 95 percent of the time. Because these venturis are smaller than the ones in an equivalently appropriately sized two barrel carburetor, fuel atomization is more efficient at lower engine speeds. The two barrel has to compromise sizing a lot more to accommodate both idling performance AND wide open throttle and so tends to excel at neither. Of course this swap to a four barrel doesn't work if you stomp the throttle all the time. This applies mostly to V8 setups. People running smaller engines tend to get rather excellent mileage and performance out of some two barrel carburetors often referred to as "Weber progressives" which appeared on many smaller Ford engines (among other things) in the earlier 1980's. They are unfortunately getting rather rare these days and there has yet to be really good equivalent aftermarket substitute.
- Simple modifications to improve turbulence in-cylinder exist! One by Somender Singh, proven to work on old and new engines consists of a groove in the cylinder head. The groove improves burn qualities netting you reduced emissions, massively more low-end torque, reduced engine temp, reduced idle speed, better advance setting possible for max power due to reduced knock. All in all, better fuel and power at the same time. There have been ZERO reports of this mod not working as advertised. Even engines that were mistuned on purpose get a 6hp advantage.