I get about 53 miles per Imperial gallon solo with all my camping gear on, which translates into ~42.5 miles per US gallon. This is with "moderate" speeds and not a lot of WOT or passing situations. I'm tickled to death with this on a heavy big-bore bike like the Bandit.
My '82 GS750E, OTOH, in the same conditions and roads would return ~62 miles per Imperial gallon or ~50 miles per US gallon. Smaller engine in a lighter bike uses less fuel, I assume......although you'd think it would be working harder and using more fuel. Apparently the GS was in a more efficient part of the rev range under the same riding conditions/speeds.
I really need to take the Bandit on a long steady speed run in the 5K-6K RPM range on a nice hot day to see what she'll return for mileage versus what the GS did.
I believe, at speed limit speeds, the Bandit engine is running too slow and is out of its "efficiency" range.
I also know that nice hot thin oil REALLY helps with the fuel economy on these air cooled four stroke Suzi engines.
I also feel that the TSCC design is a big help to fuel economy generally.
All in all, great engines (both of them), no matter how you look at 'em.
Dang, I'm still sorry I sold that GS.....