Poor Fuel Mileage

Check fuel pressure.
Check ignition timing.
Check spark plugs.
Check for correct tire pressure, vehicle carrying heavy loads or poor driving habits.
Check if A/C is on all the time.
Check for restricted air filter.
Check for restricted exhaust system.
Check for proper speedometer calibration.
Check compression.
Check for dragging brakes.
Check gasoline quality.
Check for cooling system running too hot or cold.