The fuel counter always show less fuel then you really got!
The game always show the wrong amount of fuel. When I started playing with RB 2014 season in the 15's game. The indicator show optimal fuel the last few laps, or at least -15 which forced me to go lean and short shift to save fuel to make it to the end. It doesn't matter after around the last third of the last lap the fuel is burned out. The only thing to avoid this is to always have at least around +10 fuel before crossing the finish line, which I do when I short shift, release the gas pedal 100 meter before the braking point and such things. I usually set my mix to 3 at the second or last straight at that point. Especially if someone is caching me.
In reality though you aren't allowed to finish with an empty tank. I hope they fix this for the next game 2016 because if they reintroduce career mode with 7 season or a least 5 and you can create an own character also hope we see the the return of the safety car and VSC. If they fix the fuel so when it says "optimal" is bloody well should be optimal, then I would have no problems being disqualified for not having any fuel in the car at the finish line as long as the counter which shows how much fuel you have is correct.
Codemasters should seriously fix this, if the game tells us we have optimal fuel then we should run out, but that is the reality sadly. I have complained to them before but nothing seem to happens.