We for a guy at work who takes pride in driving his 350Z very slowly and achieving over 30mpg.
So firstly, just ride the darn thing for gods sake
Tank capacity? I'm guessing, but there's that steel tube that goes into the tank a wee bit from the filler cap. Tank should probably have an air gap at the top, perhaps to take care of fuel and vapour expansion in warm weather/direct sunlight.
So I'm thinking you kept working at it and filled a good bit of the air gap.
But really I dunno, and nor should any of us care. 250 plus miles, bloody norra. Got 200 once on holiday, and I was worried I was gonna run out and get stuck.