why is my 2005 saturn ion using a lot of gas

Hello, The fuel pump will not affect in any way abnormal use of fuel. Fuel can be consumed more rapidly than expected due to a few different reasons. Most reasons are driving habit related -- speed of acceleration, aerodynamics of the vehicle in case loads are carried, and so on. Engine performance issues causing excessive fuel consumption can be measured using a diagnostic tool. If there is an issue measuring exhaust gases in the exhaust stream via the oxygen sensors, then the engine's processor may command more fuel to be consumed by the engine, thinking the engine is running too "lean". A dirty air filter can contribute at some small level, as well as worn out spark plugs. The best thing to do is to get an accurate measurement of your fuel economy, compare it to what seems reasonable, and then start performing testing by checking normal maintenance items and using a diagnostic tool and skilled technician to find out about the performance in general.