I'll reply for what it is worth: I averaged between 12 and 15 MPG in a standard 2005 Elise on track. Others have reported having pick up problems for the last quarter of a tank, but I really didn't. I am going to guess that a supercharged, higher HP car will average less.
Seems like at 7-8mpg and 70 liters, plan for 120 miles till empty?
1:30 at 80 mph ave lap speed and 2:00 at 60 mph (kart track) ave lap speed. Less time if you are faster, it is a cup car after all. If the cup car has a tripometer, reset it and pull in after 100 miles/160 km and see how much fuel it takes.