Air your tires up to the maximum.
In an effort to fix my TPMS light that never turns off, I tried airing down my tires to the door jam recommendations. (It should be noted here I'm running Michelin LTX M/S at 35 PSI max)
After doing that my MGP suffered horribly. I believe it's supposed to be 29 and 27 PSI for the door jam. Whatever it is, my gas consumption grew. I've lost easily a good 30-50 miles off a single tank of gas compared to what I normally get.
Try airing your tires up to the tire maximum for a tank of gas and see if you improve at all.
Just be careful with the back tires cause from driving for the past year and a half, I have noticed just a little uneven wear on the back two tires from them just barely bulging due to no weight in the back of my truck generally. If you run a big enough tool box or a shell I can't see if being a problem.