Ok after 3 years of owning my jeep I decided to see how many miles i could actually get to a tank till I needed a fill up. On fuel economy.gov it says I have a 21.6 g tank will 315 miles total. I drove till I hit 300.2 and decided that was close enough without running out of gas. When I filled up all I could squeeze in was 15.7 g The entire tank of gas I drove very conservatly shifting at 2k rmp and did not go over 55mph. My question is where is the other 4.5 gallons I have supposed to have. Any Ideas?