i usually run at 32-35psi. i have street tires on mine too
I notice you're running 31" tires, have you done anything to correct your speedometer to compensate for the taller tires? Do you have a GPS? If so, compare the speed indicated on your GPS with your speedometer at around 45 to 50 mph and see if it's off. Mine is almost dead on with the 30" tires, so I suspect yours may be slightly off. This will affect your gas mileage calculations, but not by 5 mpg (the difference between your mileage and mine). I know you can get replacement speedometer cable gear that will correct for taller tires, but most folks don't do it unless they go 32" or taller. I'd also try upping the air in your tires by 5-8 psi and see if that helps. What gears are you running? I have no idea what I have, but I'm assuming they were the stock gears for the non-tow package X. The tow hitch on my TJ is an aftermarket bolt on.