Just look at the guage on the dash lol, i know it isnt the most exact thing in the world but at least it gives you a rough idea of how much you have left.
Haha I know there's a fuel gauge on pretty much every car I think besides local go karts. I thought it would be better if there's a flashing indicator or something nagging for low fuel. Well my car goes crazy ding when it's low on fuel.
A wee flashing light when you get low would not be so distracting
The game has a pretty good idea of when you've got less than two laps of fuel left...
But it doesn't have to be a fuel level sensor It can just do (like F1 cars) some arithmetic on how much fuel has been injected compared to how much was in the tank
(Edit - in fact, maybe that's how the current fuel gauges work in all the LFS race cars )