I have tested my mileage with a GPS several times. If I am on dirt roads that allow comfortable driving no extra weight on board I get as much as 60 MPG no hot rodding. I have had heavy loads with two adult riders, full sized deer, and lots of gear on a Honda recon 2x4. These trips for deer hunting is nothing but rock crawling up hill very slow speed gaining 1500 feet elevation. The mileage is under 10 mpg in this application.
I never carry gas in the areas I hunt. I used to but never needed it so I quit taking it. If I go to a new area where I think i might be driving say 30 miles one way I will take a gallon with me. it depends on how the ATV is driven, and how much weight is on it, for my ATV.