Should be closer to 40. Its over jetted bit not by much. I think the ign advance unit is playing up as its got almost no low down poke too.
I get high 30's in the GT depending in how I drive it on a given day too. Have seen very close to 40 on a long highway run. Has taken a bit of fiddling, but those Hitachi carbs are a damn good thing when u work them out.
Interesting, I never got close to 40 but then again I never had a Bellett with original engine apart from one (now Dave's) GT. I don't think it did much more about 32-35 mpg though on a run. memory is a bit vague now on it. Dave might have an update on it.