Poor Fuel Mileage

Check for fuel leaks.
Check fuel pressure.
Check for clogged air filter.
Check compression.
Check cooling system condition.
Check ignition and valve timing.
Check spark plugs.
Check for adequate spark with a spark tester.
Check for high resistance in spark plug wires.
Check for faulty Throttle Position (TP) sensor.
Check for faulty Engine Coolant Temperature (ECT) sensor.
Check for faulty thermostat.
Check for faulty PCM.
Check for high idle RPM.
Check for malfunctioning electronic fuel injection system.
Check for malfunctioning EGR valve.
Check for brakes dragging.
Check for clutch slipping.
Check for low automatic transmission fluid.
Check for improper tire pressure.