I noticed that many race cars with digital fuel gauges start at 20.0 and go down from there. I think that should be 20.0 GALLONS (about 75.6 liters), but the game counts it as 20.0 LITERS (about 5.3 gallons). I know that nearly all race cars should be able to hold a lot more than 20 liters/5.3 gallons of fuel. 75 liters is about what the average race car holds in Project Cars (some more, some less). That may explain the need for frequent pitting in FM6.
If this is true, wow.
NASCAR runs about 20 gallon (US) fuel tanks and gets anywhere between 3 and 6 mpg. I remember hearing that F1 cars carry twice the fuel NASCAR does, and they get better fuel mileage (typically between 5-7 mpg).
I think Forza just picked a number that would keep everyone equal and races competitive. Considering the races are so much shorter than in real life.
Just a guess though.
A real-life LMP car can go about 13 or 14 laps at Circuit de la Sarthe IRL, or something like that. But I tried racing one of the Audi LMP cars, and ran out of fuel after about 3 1/2 laps. If you multiply that by 3.78 (the number of liters in one gallon), that would be about 13 or 14 laps, thus my thinking that FM6 counts liters instead of gallons.