1983 R100RS, 69K miles. New AM26 tires, proper inch sizes, stock rear. Recent carb rebuild. Top end n rings done winter 2009. Two recent trips fully loaded with camping gear were 1300 miles and 800 miles. First was the BRP + NC backroads and the second was NYS backroads with 100 miles of slab. MPG calculated over the total distance was 51+.
I only calculate the mpg over several tanks due to variations from tank to tank in how I fill it - where in the neck, side stand or center stand, etc. Nice range.
The '75 R75/6 does almost as well but that engine and those carbs really need work this coming winter. Should do better next summer.