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 alignment.
Check for restricted exhaust system.
Ensure all vacuum hoses are connected correctly.
Check intake manifold vacuum. Vacuum should be more than 17.7 in. Hg at idle.
Check air filter element.
Check spark plugs.
Check compression.
Ensure Engine Control Module (ECM) terminal voltages and grounds are okay.
Ensure fuel system pressure is correct.
Check fuel injectors for leaks.
Check fuel system for external leaks.