Hi
The fuel burn is many things..
1:Its related to the throttle, more throttle more fuel burn.
2:Each car have its own tank size, in the old version the thank size was unrealistic if you think it as Liters. they standard was 1000(As I recal) in version 4 its 100.
3:Each car have burnrate. This is just a Add off the the curent value, if tanksize is 100 one is normal, but if you use the old settings form V87(Tank size is 1000) then you can make the burnrate to 10. Then its very close to the old version.
If you run without magnets you need less thottle but you will have more equal fuel use. because the cars are more equal.
In other words you cannot calculate laps with the fuel burn, a better idea for forcing users to go to pit is using Tyre wear, here you can settup the % pr. lap.
Martin