I Did 32000 miles on a run to Magadan, Sibiera, driving around most of the Stans up to 5000m. I would guess half that milage was on dirt roads and averaged just over 30 mpg.
Just prior to that trip l took the vehicle to Diesel Bob,Ribchester, Lancashire to set up the pump as l had fitted a intercooler and upped the boost to 10psi.
I also spent 2 years driving from Alaska to Terra Del Fuago and average 28mpg in North America (Around 30000 miles), MPG dropped to 26MPG driving through Central America and by the end of the trip 83k miles in Uraguay was down to 23MPG. The fuel pump cold start lever was leaking and vibrating! l have since had the fuel pump striped down by Diesel Bob, finding some excesive wear, main suspect to be ethanol in fuel. Also the injectors needed refurbishment.
Have not run the truck yet so dont know the curent MPG but hope it back up to 30MPG!
Stephen