Below are some calculations I did for racing at Snett / Donny last year on a DD bike in order to minimise the amount of fuel I carried:
-----------"Based upon doing 20 laps (19 in the race, 1 warm up(0.5), 1 cool down (0.5)etc) at 1.95 miles = 39 miles. So putting 11 litres in at the start and with 2.75 left after the race means I used 8.25 litres.
So 39 (miles) / 8.25 (litres) = 4.73 miles per litre.
With Snett at 1.95 miles (same as Donny) and Cadwell at 2.25 miles.
For Snett, there are 12 laps per race scheduled, each lap will use 1.95 (lap length) / 4.73 (mls per litre) = 0.41 litres / lap x 12 = 4.95 litres total for race distance. But allowing for 2 litres to be left over after the race and 1 litre for warmup/slowdown, we'll need approx 8 litres in the tank to start the race.
Cadwell will be a different story, but if the races are 8 laps (they normally are) which is 2.25 miles per lap the calculations are;
2.25 (lap length) / 4.73 (mpl) = 0.48 litres / lap so times 8 laps = 3.8 litres. Add 2 litres (to be left in the tank) and 1 for warmup/cooldown etc, = 6.8 litres. Maybe squeeze this to 6.5, but again test this with racing on Sat.------------"
So basically I calculated my DD bike did 4.73 miles per litre or approx 21.5 miles per gallon when racing.

Tim