Poor Fuel Mileage

Check fuel pressure.
Check vacuum hoses.
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 EGR system.
Check oxygen sensor for silicone or lead contamination.
Check engine compression.
Check for dragging brakes.
Check gasoline quality.
Check for cooling system running too hot or cold.
Check for faulty thermostat.
Check for faulty Torque Converter Clutch.
Check for leaks is air intake system and crankcase.