Several race cars, including the 787B and at least some of the Super GT cars, do in fact have a digital fuel-remaining readout. However, most of them have at least something wrong with it, like the 787B only displaying tenths and I think hundrethds of a liter without actually telling you how many complete liters you have, or the Super GT cars I've tried that display all 9s once each digit gets to zero liters making it difficult to read.
Not sure how many actually work properly, but once you learn to read the super GT cars you can discard the HUD entirely since it shows your lap time as well(so does the 787 but you can't read the fuel unless you're really good at counting on the fly).
The readout in question here doesn't sound like fuel however. Don't know what it might be, doesn't sound like a fuel efficiency display either, Could be turbo pressure I suppose but that doesn't seem right either.
