This can be quite troublesome as its hard to get fuel to run out when you need it to and when you do run out you'll be going very slowly, however I did find a way to get the tank empty by finish everytime.
First go to 'Play - Go!' and press
to make it a custom event and change the race mode to Refuel and 1lap, press go and start a Refuel event on Belgium - 3000cc
Try to avoid being hit and get as many speed boosts and KERs (x3 boost from the blue roads) as you can and make sure you ONLY PICK UP ONE
bit of fuel as you're getting low near the first Pit stop, you should run out of fuel just before the last jump so be sure to get a jump boost (More details here - Nailed the Jump in F1 Race Stars
) off this jump and with enough lead you can roll to the finish.