It's a big heavy truck with a big'ish engine. Mileage is going to suck. But I agree with you, it shouldn't be that bad. You've made a couple of engine mods, have you checked your actual air:fuel ratio? Ask around in your club, someone must have an LM-1 or something similar. Have you used a tuner like Haltech or Jet? The highest setting will give you a tad more performance than the mid setting, but your mileage will defiantly be worse. Have you checked for proper air pressure? If you're lucky, you can find someone with a tire pyrometer to set the pressure more accurately. If you can't find one, just add 5 psi to your current pressure and see what happens to your mileage.