it can cause a variety of issues, usually if it's bad it will throw a check engine light on. No exhaust leak or funky sounds from the motor?

 

A simple item that will cause a large loss of fuel is a ruptured fuel pressure regulator diaphragm. This allows raw fuel to be sucked into the throttle body and may damage the catalytic converter. Pull the vacuum reference line to the fuel pressure regulator and sniff. If you get a strong whiff of gas, the diaphragm is ruptured.

A fuel leak can also cause loss of mileage and be difficult to trace as the fuel will evaporate on a warm engine quickly. Check for gas smell w/ warm engine, and if yes trace the leak.

good luck