Quote from S14 DRIFT :

AFAIK the cars all have roughly the same fuel consumption... it's just that the rate that fuel is used is proportionate to the power being produced..

Correct me if I'm wrong but if you're using 50bhp in the FZR, and 50bhp in the XFG, both will be doing the same MPG, although % per lap may show as different due to the size of the tank..

I wouldn't have thought so. Horsepower isn't a measure of fuel used. One car could make 50bhp efficiently and one could make it inefficiently. If one car used the exact amount of fuel required to make the exact amount of energy to get 50bhp and burned it 100% efficiently then it would use less fuel than a car that used more fuel than was required and made the same amount of energy by burning it at less than 100% efficieny.

I realise that's true in RL, I'm talking about in LFS, and if I remember correctly, that's how it's calculated.

An idea for everyone to have the same fuel (assuming <100%) the officials would change drivers while waiting in the pits for all drivers, that way they can see how much fuel is in the cars, and then make the pit lane off-limits (tyre wear shouldn't be an issue, people probably wont be going fast enough for serious damage).
Oh and if this actually happens, and can maybe be arranged to be a 30 minute race I'd be interested.