I put an EVO 6 fuel tank in the GSR to fix the fuel surge problem which I believe is the same tank as the 4 and 5.
Recently I reverted from E85 to 98 RON and had the amazingly good luck off limping into a gas station after total cutout at the top of a hill, the gas station was at the bottom fortunately.
It took 47.35L according to the voucher here and I understand there is maybe a litre or just a bit less at that point .
Considering the car won't go after that point I consider it the "fuel tank capacity" regardless of how much is actually left.
So definitely not 55 litres, probably 45 is a bit low as well as I make it 48 liters I suppose OR someone nameless has a bowser calibration problem.
Don't suspect I'll be in the position to do this check again but was an interesting exercise.