Looking at gamefiles i think i’ve found how fuel consumption work, related to engine power. It was easier than i firstly thougt!
Lights: Light use 1% of a Fuel Barrel per second. Always.
Engine Power: Engine use 5% of total engine power per second. Of course you have to take account of fuel efficiency bonus.
So, let’s say, with starting engine (+800 EP) and Maybe’s Daughter (+100 EP) and lights on, we will use 0,45+1= 1,45% of a fuel barrel per seconds.
With a Compulsion (+3500) and nothing else, we will use 1,75% of a fuel barrel per second, 2,75% with lights on…
And so on.
It looks clear that everything above the Boadicea, without proper reduction (5% from Tireless Mechanic and 10% from Milebreaker) would be VERY expensive.