tough question. It really depends on how much driving you do a day, what type of driving you do, etc. I typically do about 70 mi/day when I'm down there, and I'd say I use 5-10 gal per day. I drive a v6 tacoma on 35's, and I don't hammer-down at all, I'm really easy on the gas.
I've seen guys burn a good gallon on a single obstacle before. There's too much to factor in to get a good response. I think your best bet is to wheel similar trails then see what your rig does on gas.
I seem to use around 5 on most Moab trails, including driving to the trail. On long trails like Triple Threat I might use closer to 10. I also let my jeep idle a whole lot... Don't tell the hippies
I normally use between 4 to 7g in a day of wheeling. Even for long trails. Last time I did Prichett and the long way out behind the rocks I believe I only used 6.something gallons. Jeep 4.0 w/3spdA/atlasII/42s. I only use 2 or 3 on a short trail/day.
Funny thing about this. With my tired old carburated AMC 304, it always took 8 gallons to fill after a trail. Didn't seem to matter how long or short. Now with a more efficient, less tired TBI 350, (same gearing, tires and transmission) it still takes 8 gallons to fill it after a day of wheeling. It's kind of a joke my wife and I have when we pull into the gas station to fill it up after the trail... How many gallons do you think it will take? Any guesses anyone???
In my Sami I used a lot 45 horse and 37s = high rpms all day .. Also I floor it and pretend like I'm going fast on every strait/sandy / smooth section of the trail
7 Gallons to do Pritchet Canyon, Hunter Canyon, and Behind the rocks and then all the way back down to and along the Old highway, in to town, and back to the Pritchet parking lot to get my tow rig. About 28 miles total, I think. 5.3, Th350, atlas 4.3 and 40's. I only had a 10 gallon tank and I was a little worried, too. But it's plenty for a day.