Driving style makes a difference. During grinding the b-spec Indy 500 with the FGT, I found that without any instruction and identical set-ups, drivers with a cool mentality managed to get about an extra lap worth of fuel than hot mentality drivers.
Driving style absolutely makes a difference. Compare fuel usage of Bob in the B-Spec Formula GT race with the AI fuel usage. Or just compare fuel usage among the AI drivers, for that matter.