Poor fuel economy can be caused by a multitude of problems. Driving habits and enviroments play a significant role in fuel economy. Prior to diagnosing this symptom, you should drive for at least two full tanks of gas to ensure there is a problem rather than an intermittent condition, such as mountainous driving. Poor fuel economy is usually the result of lack of routine maintenance.
You should start your diagnosis by determining what service has been performed on the vehicle. Routine spark plug replacement is necessary to maintain good fuel economy. Spark plugs provide the ignition source to initiate the combustion process to burn the fuel. If the spark plugs are worn they will not provide a good ignition source for the fuel and can degrade the fuel economy. Dirty or improperly maintained fuel system will also degrade mileage. There are many additives sold for routine cleaning of fuel systems. While these are beneficial, these are not capable of complete system cleaning. This must be done by a professional with proper equipment. The air filter filters the air that enters the engine. It is designed to remove the harmful dust and debris that would otherwise enter the engine. An air filter that is only partially plugged can restrict the air entering the engine and reduce the fuel economy. Inspect the air filter and replace it even it is only moderately dirty. A new filter can go a long way in improving fuel economy. Do not attempt to blow out the filter with compressed air. New filters rely on small fibers to "grab" the incoming dirt particles. Blowing out the filter will remove these fibers and reduce the filters effectiveness. Tire inflation should be checked. Low tire pressure can reduce fuel economy drastically. Tires provide the necessary traction to prevent the vehicle from skidding and sliding. At the same time, tires provide a low rolling resistance so that the engine can easily propel the vehicle. If the tires are low, the rolling resistance is increased and will require the engine to consume more fuel to generate the same amount of power. You should ensure the tires are inflated evenly and to the proper pressure. Proper ignition timing is critical to maintain good fuel economy. Most newer vehicles have computer controlled ignition timing. On these systems, the timing is not adjustable. On vehicles that have adjustable ignition timing the timing should be checked. You should consult the under hood emission control label for instructions on ignition timing adjustments. Engine oil can also affect your vehicles fuel economy. You should ensure you are using a multi-viscosity oil as recommended by your vehicles manufacturer.
With today's fuels, good maintenance practices are a must. With the introduction of Ethanol fuel mileage suffers quickly. Attached is a short clip of an example service to help keep your mileage at its peak. For more information, gives us a call and we will be glad to help.