Honestly Snobovike, for anyone who uses their ATV just sporadically, gas mileage really doesn't matter. And like you've said, and I'll be doing this; really long rides where there are no fuel stops brings up range issues.
I'm planning rides where packing extra fuel is necessary. One is the Magruder Corridor starting from Elk City Idaho and riding to Darby Montana. Another is from Superior Montana to Pierce Idaho, overnighting there and then riding back. That one is 120 miles one-way.
I can imagine some overnight camping trips where there's isolation as well.
Oh, I've thought about riding sections of the Arizona Peace Trail. Depending upon the sections I choose, extra gas would at least keep range anxiety down.