Better yet, park and go inside. A low-viscosity synthetic can improve your gas mileage by 3 percent. Every one mile per hour over 55 MPH decreases your gas mileage by 1-2 percent. Avoid sudden acceleration and braking, and use your cruise control whenever possible. Clean out your car.
Excessive weight harms gas mileage.
Each 100 pounds in your car decreases gas mileage by about one-half mile per gallon. Replace your spark plugs on time, and replace them with high-performance spark plugs, such as Bosch Platinums, and gap them properly. Platinum plugs don't wear out as quickly, and while the jury is out whether platinum plugs inherently give better gas mileage, a set of old platinums will have an edge over a set of old cheap plugs. Two bad plugs can decrease your gas mileage by 20 percent. Use your air conditioner on the highway, since open windows increase drag.