[Piotrsko]

Just checking: it's a passenger van, not some weird excursion variety. So aero is your devil. Brick wall in the front, huge vortices behind you, the side mirrors are probably worth a 2mpg penalty just by themselves.

80 psi tires means 6ply or better, probably another 1or 2 because they are heavy and not designed for fuel efficiency or low rolling resistance.

You're mantra should be clean up the flows around and below the vehicle. The people here know their stuff, welcome.

And no reason you can't improve the mpg. Wont be 30, but it will be better than you get now.