Hey,

I had this issue serveral times now in iRacing. If I ask to calculate fuel for, let's say, 10 minutes in a GT3 car on, for example, Zandvoort (Full), I get answers like "We calculate 382L of fuel". If I simply ask "How much fuel to the end", I get realistic results, even though it uses the same amount of time, when the race is only going for 10 more minutes.
Had the same in a Skippy the other day, on Okayama. I asked to calculate fuel for 18 laps or so and I got an answer around 18L. So I took 19.3L or something like that and still had a 2-digit amount of L left after the race. I asked this question during the end of qualifying with 3 driven laps. Shouldn't that be enough?