I've been looking what fuel consumption I get for what gear spacings ever since reporting this. Now I hope this can be helpful in tracking down the problem:
I've managed to break all (my) records in fuel consuption with this car:
I'd not even call this a car anymore, this is a fuel eliminating machine! It eliminates an amazing 1400+ l/100km (some 0.15 mpg), meaning it would need at least two refuels in order to complete Green Hell...
Drag at 80 km/h is something like 10kW (it's hard to exactly read out the graph). In both gears it would have more than enough power to make those 10 kW. So it should drive in first with something like let's say 1000g/kWh. For 100km it needs to drive for 1.25 hours at 80km/h. Now I simply multiply all that up to 12.5kg/100km. Specific weight of fuel is araound 0.75kg/l, so I divide by that, resulting in 16.667 l/100km. Not 1400.
So the game calculates 84 times as much as I do, so it can't possibly be because I misread drag or fuel consumption (I would have to have misread both by a factor of 9+).
Now I wonder what is the difference between my calculation and the one in the game, but clearly there is some difference.
Edit: Maybe it's driving in 2nd, which would explain some of this and maybe drag is considerably more... ...if I'm not mistaken I could get the drag coefficient out of the car's lua and try to perform the whole calculation...