Personally, I've been shocked by the fact that my tuned Daihatsu Move (730 Kg, 180 hp, 4WD), after a dozen laps at Tsukuba, used up a bit less than 1/8 of its fuel tank, and when I entered the pits, I was able to refill up to 12 liters of fuel, which means it's probably got a 120 liters fuel tank (if 1/8 = 15 liters, 15*8 = 120 liters).
I don't dispute the Veyron fuel consumption (seems about right; fuel tank size is too, although in real life it's got a 100 liters tank), but other cars (especially small ones), when fuel consumption is activated, appear to have an extremely huge tank, almost impossible to deplete. By pitting in to refill consumed fuel and doing a quick calculation, it's clear that PD haven't fixed this issue since GT4 (actually they made it worse in some ways).
Large racing fuel tanks could have been implemented in the game a component of racing modifications.