If fuel consumption is on in a race, go into the tuning menu and look at the fuel after you have completed a couple of warmup/practice laps. If you do more than one lap, it'll give you an estimate of the number of laps your fuel load will carry you. Stock is 30 litres and in a lot of cars it doesn't take you far. I've run races of 35 laps where I had to carry 80 litres of fuel. You can adjust the fuel load in that menu and it'll give you an estimated number of laps until you run out of fuel. What I do is add about 2 laps worth of fuel more than the calculation just to be safe, then save it as a separate tune so I can access it quickly if need be.