Horseman, you are correct...memory back on-board...that the Norton 850 Commando Interstate model had the larger fuel tank. The guys I rode with back in those days, one had the same bike as yours...a black Interstate, while another had a black roadster, and I had the red roadster. It was nice to have the larger tank, but they were an ugly addition to the bike. I liked the looks of my smaller roadster tank, but had to stop for fuel more often.