Make sure that you change the oil and maintain your car’s powertrain according to your owner’s manual. This will ensure that your car’s engine will operate at maximum efficiency, thus providing the best fuel economy. Dirty air filters, old spark plugs and low fluid levels can affect engine performance and fuel efficiency. By keeping your car in top mechanical shape, it will run better, last longer, and return better fuel economy. Follow the manufacturer's maintenance schedule. You can check the air filter by holding it up to a bright light. If it looks clogged or obviously dirty, replace it. Keep tabs on the fuel filter, also.