I saw this on lifehacker yesterday and it's been bugging me since. In the post, "Increase Your Mileage. Save Gas", they suggest four ways to reduce your vehicle's fuel consumption: (1) Slow down, (2) Skip the AC, (3) Inflate your tires properly, and (4) Accelerate Slowly. Being a car geek, these over-simplifications bug me, so I dove into the original article, "Tips to Increase Your Car's Miles Per Gallon" to see if it did a better job of explaining "why". It doesn't, so I'll give it a shot.
First a little background (which has to be a bit simplified or it would be a book. Sorry.) There are exceptions, but in general, modern fuel injection systems are load (typically measured by MAF or MAP) and RPM based, and the amount of fuel injected typically increases with both of those metrics. Fundamentally, it has nothing to do with vehicle speed, the AC being on, or your tires. The fuel injectors spray however much fuel is needed to hit a target AFR (air-fuel ratio) for a given engine load and RPM.
Knowing that fuel injection rates are tied to engine load (and a few more details that I'll try to throw in) let's go over this list and explain why:
(1) Don't accelerate quickly.
Accelerating hard wastes uses a lot of fuel for a few reasons. First, you've increased engine load; Second, the car is going to richen the mixture to reduce the potential for detonation as heat and load builds; And finally (and this is where it's going to get messy) you might have switched the fuel injection system from Closed Loop to Open Loop.
In closed loop fueling, the car's computer (ECU) is watching the Oxygen sensor and tuning the fuel trims automagically to target a stoichiometric air-fuel ratio of approximately 14.7 parts air to fuel. This ratio is generally agreed to be the most efficient at burning all the available fuel, and it's also the ratio for which catalytic converters are designed to operate most efficiently. However, the car doesn't always try to be a non-polluter. In my Subaru, for example, the factory ECU switches from closed-loop fueling to open-loop fueling at approximately 60% throttle (TPS). If you press the gas pedal past this cut-over point, the car no longer targets a 14.7 AFR, but instead switches fueling for maximum power (which in a Turbo car is anywhere from 12.5 AFR to 10.5 AFR depending on the quality of fuel -- and yes, the better the fuel, the less you need.)
(2) Let your car brake itself.
It's arguable whether or not engine braking is a good idea mechanically, but it's generally true that the car needs less fuel under this condition. The reason, once again, is because of reduced engine load (ie., the engine is no longer working to push the vehicle, thus it's no longer sucking in a large volume of air, thus it no longer needs to spray a lot of fuel to keep that large volume of air at a particular AFR.) The engine may even use less fuel under engine braking then it does when coasting in neutral. That said, the brakes are designed to slow car, not the engine, and the brakes are cheaper to replace, so this isn't always a good idea.
(3) Drive at the speed limit on highways and freeways.
Speed has nothing to do with fuel consumption. It's load vs. RPM. The simplified reason your car uses less fuel at 55 MPH then it does at 60 MPH is because if you stay in the same gear, RPM's increase with speed (as does aerodynamic drag, which increases load.)
Of course, this rule doesn't always hold true. If you can draft another vehicle, you'll reduce engine load. Furthermore, gear selection plays a role. 55 MPH in 4th gear will run at a higher RPM and should use more fuel then 60 MPH in 5th; But 55 MPH in 5th gear might not produce enough torque to climb a hill, thus increasing load when you try to maintain that speed without downshifting. You can also sometimes run the car leaner at high RPM's after you've past peak torque output, so increases in RPM don't always linearly add fuel.
(4) Use cruise control.
The benefit of cruise control is it's ability to be smooth. If you can avoid sudden load, you'll need less fuel.
(5) Don't use the air conditioner.
First of all, don't die of heat exhaustion just to save fuel. If it's 120F outside, you should probably think of your immediate health first. That said, this tip is good at low speeds since running the AC compressor raises engine load. However, this tip doesn't always hold true at highway speeds where rolling up the windows can reduce aerodynamic drag.
(6) Accelerate before hills.
Mmmm... inertia. Going uphill raises load (or climbing stairs wouldn't be exercise.) Accelerating early may allow inertia to carry you up the hill instead of asking the engine to do all the work.
(7) Clean out your car.
The key point here is to lighten your car. It takes less power to move a lighter vehicle, and less power == less load == less fuel (or quicker acceleration on the same fuel.)
(8) Check your tire pressure.
Low pressure increases rolling resistance and increased resistance == load. Unless you're driving in sand or rock-crawling where low pressure == traction (and you're not going fast enough for it to matter.)
(9) Change your air filter.
If your air filter is clogged the engine will have to work harder to suck air through it. Working harder == increased load.
(10) Get a hybrid car.
Uh..
(11) Do more in one trip.
Doesn't save fuel per mile, but miles per day, which is also a good thing.
In summary, if you really want to burn less fuel... reverse-engineer your ECU, study the fuel maps for your car, and drive accordingly ;-)