Hi ramyankee -
2006 5.9 Cummins with 147,000 miles, 6spd trans, don't remember rear ratio - on the weekend I hauled a 39ft x 13ft high tri axle toyhauler 100 miles round trip, GC wt 24,000lb. 24 miles in 30 mph crosswind, 38 miles 30 mph headwind, 38 miles 30 mph tailwind. Rolling country with 3 or 4 hills about a mile long guessing 6% to 8%. I had to shift to 5 to hold 55 mph on the steepest hill, against the wind. The rest of the trip was on cruise, 65mph/2100 rpm in the big sprocket. Trip meter registered 12 mi/US gal for the round trip.
Not sure how an 8.1 gas would compare in the same conditions.
