I really think you need to look at the course, type of terrain, and weather and gearing. I have raced my 200 with (stock tank), which is jetted pretty fat. I ran 13/51 gearing with a Maxis IT120/100/18.
In enduros it depends on the format....I have got right at 50-55 miles out of a tank, but that was a time keeper, the "new format" qualifier type, I've needed fuel and have topped off my tank 45-50 mile mark. The qualifer events I seem to use more fuel..... special test sections???
In D-36 our hare scrambles /cross country races tend to be more open terrain, which means higher speeds, more main jet, more fuel! I have been able to go the distance on a full tank. I topped off to the tip top. I keep my fuel can in the pits with 1 gallon in it, no more or less. If I do need fuel it's on the second to last lap or the start of the last lap. The reason I use 1 gallon is it will not over flow, empties the can quick, and will be enough to finish the race if I make the time cut off and have to do another lap I have enough to go the distance.
I always asked the club how long the course is. Then I looked at the results of the race(s) that went before me. I count their laps and lap times, that gives me a pretty good idea on time / laps & miles, then I would figure out my own game plan, most of time I could go the distance. I have never ran out of fuel, but I have had to go to reserve, just once.
I would look for a loop that you know the miles, then top off your tank and run it at race pace for 2 hours and see what happens. That should give you a good idea on how to plan.
Hope that helps or give you food for thought.
i use a 5 gallon jug, and i add 2 gallons to this jug when i think it needs it. so i've not had an empty jug, or a known quantity in a long time..
