I drive a 00 350 psd with 800 pounds in the back all the time K&N intake & straight pipe exhause super chip tuner @ 55 to 65 I get about 18 to 20 mpg if I go over 70 & stay in the turbo drops to about 13 to 16mpg loaded or not
My 02 crew 4x4 with 35's only gets about 17/18mpg on a good day empty @ 65mph (superchips tuner on tow mode). At 75 it does drop to 15/16. Pulling the YJ it drops to 12/13 depending on the terrain.
Our 02 PSD 2WD crewcab dually w/6spd gets about 16 loaded with a heavy trailer, and a best of 22 highway, empty, and driving like an old lady. Normally 17 city/19 highway
With a slighly lighter SRW 250, and an auto trans, you may get 1 or 2 MPG better
my 99 350 with the preditor on tow, 4" exhaust, Afe filter, 35's, and empty gets 16-17 if its all highway. Put it in town, turn it up to 100 or pull the rig and it drops to 13 never went below 13 even at 24,000 lbs. and change who new 2 broncos were so heavy