4. Junk in the Trunk

Hauling around paraphernalia — books, training weights, broken car parts, sand for winter traction — is wasteful because accelerating that ballast consumes fuel. The Department of Energy estimated that excess weight can impact mileage by a negative 1 or 2 percent.

When Car and Driver editors piled 775 pounds of ballast into a 2,000-pound small car, mileage fell by 1.5 mpg. The U.S. Department of Energy says 100 pounds of cargo is enough to adversely affect gas mileage.