Hi guys, just want to gather some information. My fc can fit about 12 gallon, but when im goin to refuel im shock that it only went to 120 miles. From what i know it supposed to be 17/24 miles per gallon. I change to catback exhaust. Thats only what i did. What could be wrong? Plz help

 

One more thing, my mechanic said my engine compression is bad, thats my fuel efficiency is really bad. And the only way to fix it is change the engine. What do u guys think about that?