well if thats the case i would leave the engine stock. just make sure to keep up with maitanence. air filter, plugs, wires, o2 sensors, anything that affects the performance of your engine. as well check your tire pressures (more pressure will help milage but may poorly affect tread wear), make sure you dont have any seized calipers, check alignment. also you can drive like a grandma, or sell your v8 powered hunk of ford and by a honda civic.