boxbox is a simple app for Assetto Corsa that calculate the fuel consumption during a race and will show a warning when you need to pit (i.e.: when you have just enough fuel to do 1.2 laps), and will tell you how much fuel you need to finish the race.
If you pit early boxbox will let you know if you have enough fuel to finish the race.
Note that the amount of fuel displayed is the total amount you need, including what's left in your tank. Boxbox will only run during a race session, and unless it needs to display something it only shows the title to prevent wasting screen space.
Here's how it looks like:
To install it download the zip file, unzip the 'apps' and 'content' folders in your main Assetto Corsa folder. Remember to active the app in Assetto Corsa: Options -> General -> UI modules.
In game you can then activate it by clicking the fuel tank icon: