Chevrolet B16, the reward car seems to have unrealistic mileage. The game thinks that that this 1970s Chevy race car can run 36 laps of Le Mans flat out.
It’s impossible to compare directly but yesterday, the prototype cars were running 14 - 16 laps per stint. 36 is insane. This is a 1970s cars. 36 laps is 305 miles flat out.
doing the math based off of a similar engine in a personal racecar I own, yeah it would roughly need to have a FORTY gallon tank to achieve that, assuming its a similar 7.5mpg than our car gets.
Also, not a chevy, its ford or bmw engine 
It is hard to find fuel tank data on the B16 online, but from what I could find, you can fit tanks in both ‘wings’ of the car (beside the cockpit). I’ve also seen someone mention that a Lola T70 (car of similar style and vintage) has a 50 gallon tank, so 40 gallons doesn’t sound unreasonable for the Chevron with that context.
The stock engine is a 2.0L BMW M10, which even with some tuning would have been fairly fuel economical for the time.
I think we need to see some figures that were used in building the car for Forza, before determining how accurate the fuel consumption is, unless someone with an M10-engined B16 is able to chime in.