Keep the wheels aligned
When a wheel or two is out of whack, your car is covering less ground for the same amount of effort. Hitting bumps or getting into fender-benders can knock wheels out of alignment, causing them to pull to the side and drag down the rest of the car. That forces you to hit the gas harder in order to maintain the same speed. Have your alignment checked at least once a year, and get it fixed right away if you feel your car pulling to one side.