Get an older cummins. My buddy marty (440nuts if still registered) has an 04 with the 5.9 5 speed full 4 door cab K&N filter charger, quadzilla power adder, synthetic oils everywhere and he knocks down 26 in the summer and 32 in the winter. The motor likes cold air a lot. Flat land at 55mph I have seen it make 36mpg with 40 degree air. Pulling a goose neck trailer rated at 16K loaded with TBB he knocked dowwn 17mpg in the summertime (95+F) he can pull my bumper pull trailer (about 1600lbs) and not change his milage. We went to get the 1980 dodge stepside truck from member showtime in North Carolina something like 800 miles adn I bought three tanks of fuel (one there, one back and one to replenish for his time). I would stay away from the newer ones with 6.7s because a friend that works (parts manager) at the nearest dealership told me they soot up turbos for excessive idling (cure is brake fluid into the intake stream cost 300 after first time) and the motors do not get the milage the 5.9s do.