On trail fuel economy can be affected in LARGE part by how you wheel and how hard you drive.
My FJ60 with v8 gets me 16.5MPG on the hwy going 70 and loaded (7000lbs). Going 85 I get more like 12 or 13MPG. Everybody knows the differences there... no real shocker. So, with my 40 gallon tank that's anywhere from 630 to 450 miles on a tank till I am sitting on about 2 gallons in the tank left. Not a bad range one might think even with the hammer down...
On the trail?
I typically see 9-10MPG if I am doing casual expedition type travel. The type where you cruise along at 30mph or so and have an obstacle every hour or so you might have to slip into low range for... BUT... if I drive like I am "trying to catch up" my economy can drop to as low as 6-7MPG.
So, that means my range goes anywhere from 260 miles to 380 miles depending on how I drive. So, my point? know what your range is based on how you drive, and drive appropriately so you don't burn a ton of gas when you might need it later. Also, a heavy foot WILL use a lot more fuel. Driving smoothly (not necessarily slowly) will help mileage off road just like it does on road.