My 75 MK3 has it's original Interstate tank and it holds just under 24 litres, the number Les provided. To my eyes this version is slim enough to look good and still holds lots of fuel.
I did a 230 mile trip on Sunday and filled up afterward, 18.3 litres. This works out to approximately 57 miles per Imperial gallon.
So the range with about one gallon or 57 miles remaining for safety is 250 miles. Run dry would be right around 300 miles.
Last summer I met a fellow who tours on his 1974 Roadster. His wife rides pillion and the bike is loaded with camping gear. They have run the bike out of fuel at 90 miles, so now start looking for fuel every fifty miles. As he explained, you start looking at fifty rather than at 80 because there might not be any fuel available at 80 or even 90 or 100.
Seems like he needs an Interstate tank. All things considered it would increase his range from 50 to 250 miles or thereabouts.
Glen