Aggressive driving (speeding, rapid acceleration and braking) wastes gas. It can lower your gas mileage by 33% at highway speeds and by 5% around town.
Vehicle reach optimal fuel economy at a different speeds (or range of speeds), gas mileage usually decreases rapidly at speeds above 50 mph. You may assume that each 5 mph you drive over 50 mph is like paying an additional $0.25 per gallon for gas.
Avoid keeping unnecessary items in your vehicle. An extra 100 pounds in your vehicle could reduce your MPG by up to 2%. The reduction is based on the percentage of extra weight relative to the vehicle's weight and affects smaller vehicles more than larger ones.
Hauling cargo on your roof increases aerodynamic drag (wind resistance) and lowers fuel economy. Rear-mount cargo boxes or trays reduce fuel economy by much less.
Avoid excessive idling. Idling can use a quarter to a half gallon of fuel per hour, depending on engine size and air conditioner use. Turn off your engine when your vehicle is parked.
Use overdrive. When you use overdrive , your car's engine speed goes down. This saves gas.
Don't smoke. No you won't save gas but you will save money and your car will smell better. :)