Wait what? How are you getting 50 litres in on a fill up? My GS has a 50 litres tank, and I get 37 to 42 in from around 1/4 or less. Do you wait for the light to come on then drive 50 kms more? I think I get about 450 kms between fills. You should still try a couple more samples to average out any vagueries of the pump.

You might want to check the accuracy of the pump. Perhaps the station is scamming you? I still can't ever see pumping 51 litres. That's weird IMO.