A car consumes most gas as it accelerates. It's a simply law of physics (force equals mass times acceleration). A moving car doesn't require much gasoline to keep moving (due to the inherent inertia). In real life this means, in order to improve your mileage you need to keep the ride smooth. Let me give you some examples.
About 30% of the drivers I see in somewhat heavy traffic apparently cannot control their speed with the accelerator pad alone. Instead I see those guys speed up and slam on the brakes all the time. Obviously, that makes the guy following too close behind very nervous and he too needs to brake and accelerate constantly. In really heavy (but still moving) traffic about 90% of the cars do this. It is relatively easy to hold a speed in a long line of cars without stepping on the brake. Just keep a little bit more distance and try to practice this. If the traffic moves along, you rarely need to brake, unless everything slows down. If you pay attention to the cars ahead of you (not just the one right in front of your nose, but the other cars ahead of that one), you can anticipate when things will slow down and you can ease off the gas. This means you won't lose all that power to friction (on the brakes) and you can keep your speed without having to accelerate. In heavy traffic this is the most efficient way to save gas and can easily get you 10% - 20% better gas mileage.
