Poor Fuel Mileage

Check for fuel leaks.
Check fuel pressure.
Check for clogged air filter.
Check compression.
Check cooling system.
Check ignition and valve timing.
Check spark plugs.
Check for adequate spark with a spark tester.
Check for leaking or loose spark wires.
Check for faulty throttle position sensor (if equipped).
Check for faulty coolant (water) temperature sensor.
Check form thermostat out of order.
Check for faulty ECM.
Check for high idle RPM.
Check for malfunctioning electronic fuel injection system.
Check for malfunctioning distributor mechanical or vacuum advance (if equipped).
Check for malfunctioning EGR valve.
Check for brakes dragging.
Check for clutch slipping.
Check for low auto transmission fluid.
Check for improper tire pressure.