Racing in the S class, i always need to add some fuel at the pitstop. My first 2 races is just went for the amount of fuel the computer was giving me and i always came short on fuel somewhere in the final lap. Am i the only one that experienced this? (maybe it's related with the tyre wear too, as the car seems to consume more fuel and tyres after a pitstop)
It depends on the car and how you drive it. I have ran out before the line twice by accident and once on purpose. The AE Viper is hard on gas, on Willow Springs I had to add more fuel to it than any car I have driven so far and still ran out coming down the final straight. On the other hand The Shelby Series One and the RUF BTR can complete the race without adding any fuel.
In general though if you have a habit of going to lower gears than needed and high revving a lot then you will use more fuel than you would otherwise and this can cause you to run short. For example if you always go to the suggested gear you are likely to use more fuel than you should as that indicator will often tell you to go to a lower gear when you really should not do so.
I've only had this happen in the timed races... I lapped much faster in the second half of the race and, thanks to the scripting, had the AI right behind me at the end of the race. I couldn't slow down to avoid doing another lap... and so cruised the last half a lap. Fortunately the AI all stopped in turn 1. Very strange, my erratic driving performance must have screwed the whole game