Nearly 20% of cars on the road have a loose gas cap. The Car Council says an unsealed cap allows gasoline to vaporize, reducing your mileage by up to 2 miles a gallon. That's as bad as under inflated tires, dirty air filters and worn spark plugs.

Always twist the gas cap until you hear it click. On older vehicles, check for a  tight seal.

Driving 55 mph saves gas, but as long as you keep your speed at 65 or under, you'll save fuel on a trip, according to Edmunds.com.

On dry roads, use cruise control to maintain a steady speed. On frozen roads, don't use cruise control because you'll want to speed up and slow down as conditions and traffic allow.