Federal Trade Commission Tips: 1.) Keep your tires properly inflated. It can increase gas mileage up to 3%. 2.) Stay within posted speed limits. Gas mileage decreases rapidly at speeds above 60 miles per hour. 3.) Remove excess weight from your trunk. An extra 100 pounds in the trunk can reduce a typical car's fuel economy by up to 2%. Lastly, keep your car in "shape." Keep your engine properly tuned and use the recommended grade of motor oil.