Slowing from 65 to 55 mph may boost your fuel efficiency by 15%; cut it to 55 from 70, and you could get a 23% improvement.
The faster you push your vehicle, the more aerodynamic drag reduces engine efficiency. For every 5 miles an hour over 60, you reduce your gas mileage by 5 to 10 percent, according to the EPA, which means more pain at the pump. A key point is to avoid aggressive driving (speeding, rapid acceleration and braking), which can be a big waste of gas in any situation. It can lower your gas mileage by 33 percent at highway speeds and by 5 percent around town. This isn't to mention the fact that keeping down the road rage is safer for you, those who ride with you and everyone else on the road. Remember also that driving significantly more slowly than the highway traffic around you can also be dangerous, so keep in the right-hand lane and be aware of other motorists.
Of course, one of the best ways to conserve energy when operating your vehicle is to make sure you don't leave your car idling. Shut off the engine if you are waiting for passengers, stopped at a train crossing, waiting for a drawbridge to be lowered or in similar situations. In particular, don't idle when you're picking up or dropping off children at school--this will lower kids' exposure to exhaust. NRDC's Luke Tonachel provides more fuel-efficiency tips in "How Can I Get Better Mileage Out of My Old Car?" on Simple Steps.