I found that in our very open terrain HS races (jumps, whoops, berms, no trees anywhere, basically a long MX course), I burn about a gallon an hour, and speeds are about 30mph. So I could probably do it on the stock tank. Faster people burn more fuel than I do though, and our races are at 5000 ft ASL, so the bikes are jetted leaner and are less powerful, probably getting better mileage. If your course is really slow and tight, lots of wheelspin, sand, etc. you might not make it. If I were you? I would go buy one of those 1 quart plastic gas jugs that take the place of your handlebar pad. I used those before, they work well, give some padding, and are cheap and quick to fill your tank. Then you should be covered if you do run out. I think Acerbis or Maier makes them.