In the summer of last year, I managed once to get 260km (161.4 miles!) out of a full tank.
This was a mix of normal riding, around the beautiful Edersee lake district in Germany, and the ride home on the Autobahn A7 at 140 km/h.
I've not managed that again since then, but consistently get at least 200km/120 miles out of a full tank.
I rode down to Monaco from here near Hannover, via Montreux, Geneva, Grenoble, Gap, last autumn, stopping for fuel about every 180-200km, which is about the time I want to stop anyway on long tours.
I don't need a bigger tank.
PS, the picture was taken in Apricale, Italy. Just 40 km from Monaco, beautiful ride up through the south-west end of the Alps.
Through the mud & the blood, to the green fields beyond...
British Army Veteran 1975-1984