Find the following line in your "...rFactor\UserData\<player
name>\<player name>.plr" file:

Relative Fuel Strategy="0" // Show how much fuel to ADD, rather than how much TOTAL fuel to fill the tank up to

Change the "0" to a "1", and you will see in the garage that a "+" sign appears next to the amount of fuel for each stop. That amount of fuel will be added when you pit.