the first thing you should do is make sure it runs right!, i guess u already started doing that. check and change the air filter, spark plugs and if u had oil on the outside of it then its because of putting too much oil in the engine, and that results in oil fouled spark plugs. check all your fluids. check your fuel pump and fuel filter. if one or both of those a bad then that results in bad gas mileage. check the tire pressure make sure its right according to the label in the door jam. that should help a little