Poor Fuel Mileage

On M/T models, ensure clutch is not slipping.
On A/T models, ensure transmission is not slipping.
Ensure brakes do not drag.
Ensure tire size and air pressure are to specification.
Check for restricted exhaust system.
Ensure all vacuum hoses are connected correctly.
Check intake manifold vacuum. Vacuum should be more than 17.7 Hg at idle.
Check air filter element.
Ensure ECM terminal voltages are okay. See PIN VOLTAGE CHARTS article in the ENGINE PERFORMANCE section.
Check ignition timing.
Ensure fuel system pressure is correct.