Got good clean filters ? Tyre pressures right ? Car been serviced by a specialist diesel mechanic ? Got the right tyres ? You'll do better with a set of 6 or 7 inch x 16 inch rims with 235/85r 16 all terrain or HT tyres than you will with 35" mud terrains.
Do you drive like a Nanna ? Google driving to maximise fuel efficiency ?
Well I do have the marks .6 overdrive in my ute now, with 4.1s and 35s.
Does 1770rpm at 100. Revs way to low, not enough torque and just to much load for decent fuel economy.
I found best economy I ever got was 285s with the 4.1s.