No, I'm not saying you go get a Prius—that's cheating. Priuses (Prii?) are not only not as green as some think, they almost never pay for themselves in fuel savings, despite the awesome MPG.
I'm also not going to focus upon what you can buy to improve MPG. There are several products that can and will help, but nothing compares with driving behavior. Yes, I highly recommend a low-viscosity premium synthetic oil (like the fantastic new Pennzoil Platinum Pure Plus in 0W-20), but I'm going to focus on driving style.
The reason for this is that driving style more than all other factors explains the differences in fuel economy from otherwise identical vehicles. Large trucking fleet operators will tell you that the differences in MPG from one driver to another can be 30% using the same truck. Many fleets have incentive rewards that allow their best MPG drivers to drive faster (making more money per hour at the same rate per mile). After all, they can get better MPG at 65mph than another driver may get at 62mph.
It really boils down to two key ideas: how the engine makes power, and what happens to the energy it took to make that power after it as been used. Since the latter is the simplest, let's knock that out first.
Dude, Where's my Energy?
Stay off the brakes. Period. It's that simple. Every time you touch the brakes on a regular car, you are converting the kinetic energy of the car into wasted heat. Where did that energy come from? Hint: it's stored in a tank in your car, and you stop at stations to buy more at crazy high prices. Obviously, you must use the brakes to drive safely and obey the law. But you should absolutely take every measure to ensure you are only using them when needed. Kinetic energy goes up with the square of speed; if you double your speed, you have 4 times more kinetic energy. Three times faster? NINE times more energy. Stepping on the brakes to stop from 90mph will dissipate 81 times (!) the energy of stopping from a mere 10mph. That's energy you spent burning fuel and can never get back. Darn that 2nd law of thermodynamics.
Staying off the brakes means coasting. It means recognizing red lights early, and letting less informed drivers pass you just in time to slam on the brakes for the red light you are coasting up to.
Here's a rule of thumb: try to only brake below 25mph. If you can keep your "brake engagement" speed low, your wasted energy will also be low—and the MPG will be much better.
Savvier readers are probably noting that I didn't mention anything about wind resistance or tire rolling resistance. It's simply the case that you cannot change either of these factors with driving style, and they do matter far less than the factors you do have some control over. Major on the majors, right?
Let's turn now to how the engine makes its power.
Your engine wants to make power a certain way in order to be as efficient as possible. Namely, it wants to be run at high load and low RPM conditions, near the point where it will explode in a violent cacophony of metallic carnage. Near—get it? Not AT. Price is Right rules are in effect—you go as close to the line as you can without going over, or you lose. You may have been taught not to "lug" the engine, that it would damage it. This is baloney.
The talented engineers of your non-GM automobile (and the cost reduction specialists who build GMs) do their best (GM: ok, maybe they could do better) to make sure you cannot physically load the engine beyond its mechanical limits. You simply won't be able to hurt the engine by putting too much load on it. Either the engine will stall or something else will give way, so worry not about hurting the engine. Lug away all you can, because it's the way your engine wants to be driven.
Explaining why takes a little geeking on automotive engineering, so bear with me.
Fuel is power. Air is needed to support combustion in appropriate ratios, but fuel is what makes power. To make the engine as efficient as possible, we need to ensure that the highest percentage possible of potential power (fuel flow into the engine) actually shows up at the transmission as available power.
Unfortunately, there are lots of other things going on in the engine that are trying to steal away the engine's power. The engine is sucking through a throttle, having to do a lot of work to pump air through the engine to support the fuel burning. The engine is driving an oil pump to keep itself lubricated—and a "Water" pump to keep it cool. These are called "pumping losses" because they represent lost power spent pumping air, "water" (coolant), and oil through the engine.
Another class of losses is called "parasitic losses" and they involve everything else that sucks power from the engine. This is the work to open and close the engine's valves. The work to turn the alternator and refrigerant compressors is also a parasitic loss. Frictional losses, like the friction of sliding the piston's rings up and down against the cylinder bore, are also considered parasitic losses.
Conveniently enough, there is a useful proxy that represents all the bad wastes of power in an engine: RPM. Every source of wasted power in an engine goes up with RPM.
Fuel-efficient driving then become a matter of regularly achieving the lowest RPM you can use for the conditions. Because engine RPM is affected so much by the transmission, we'll discuss that next.
A great transmission can really help fuel economy by ensuring that the engine is operating in its most fuel efficient condition under most driving. If you have a manual transmission, then it's easy to drive with economy; you just short-shift and keep the transmission in such a high gear that the engine seemingly can't accelerate. If you step on the accelerator and get a rush of acceleration instead of a whimper of protest, then you're doing it wrong.
Automatic transmissions are trickier to fool into giving you better MPG. There is a way that you can, though it requires some finesse. An important fact allows you to manipulate the transmission and fool it into giving you better MPG: the upshift and downshift points use different load factors.
Huh? Well think for second about how the transmission knows when to shift up or down. When it sees high throttle inputs, it holds the lower gears (because it thinks you want acceleration). If it sees a low throttle input, it assumes you are at a speed you want to maintain, or simply aren't accelerating as fast, so the lower gear isn't needed; it will upshift to the next gear.
Based on throttle inputs and programming about vehicle speed and engine speed ranges, the transmission will either upshift another gear, or downshift back to the lower gear. Low throttle=upshift to another gear. High throttle=downshift to lower gear (as for passing situations). The thresholds of where it will shift are vitally important to fuel economy.
Pay attention to this: the transmission will shift up to 3rd gear (from 2nd ) at a much lower throttle input than it takes to get it to shift down to 3rd gear from 4th. That means you can back off the throttle to cause the transmission to upshift, then add more throttle back without triggering the transmission to shift back down to the previous gear.
So to properly drive an automatic transmission for fuel economy, you should only add throttle as you go to higher gears. Perhaps 10% throttle until you get to 2nd gear, then go to 20% throttle until it shifts to 3rd gear, then 30%, and so forth. The resulting acceleration is faster in the higher gears than many people do, and slower in the lower gears. This is the key to better MPG with an automatic.
It will take some time to develop a feel for just how far you can push the throttle in a given gear without triggering a downshift.
Driving for fuel economy is about keeping RPM low and engine load high. Lower gears take load off the engine by letting it rev up—which is NOT what you want for MPG. Shift early, shift often. Keep the RPM down and throttle restrained. Then, stay off the brakes and coast wherever possible in lieu of braking. Practice is a bit and you can easily achieve 20% or greater improvements in MPG.