Automobile Maintenance:
Keeping your car tuned can increase fuel efficiency by up to 4%. Replacing clogged air filters improves gas mileage by up to 10%. Air filters also protect your engines by keeping impurities from damaging the engine

http://www.fueleconomy.gov/feg/maintain.shtml

Properly Inflated Tires:
Under-inflated tires causes car engines to work harder, using more gas.This can improve gas mileage by up to 3%. You can find the recommended tire pressure on a tag in your car or in the owner’s manual

Reduce the Weight in Your Car:
Although there are certain items you should carry in your car, such as a spare tire, any extra weight in your car decreases your fuel efficiency. An extra 100 pounds in your car could increase your fuel needs by 2%. Keep in mind this affects smaller cars more than larger ones and the percentage of extra weight is relative to your vehicle’s weight.