Gearbox sprockets are not that expensive so good suggestion. Went up 1 tooth on the gearbox on one of my other bikes and it dropped about 400RPM in top gear at all speeds. Made hardley any difference to performance but made cruising a more laid back.

Also I believe when I restored mine I found 39 rear was stock. That’s what I have on mine. Going from 40 to 39 would make less bussy too.

Pleased to say that 1 extrat tooth on the gearbox sprocket seems to make little difference to acceleration, but does make it feel a littlle less "buzzy" at cruising speeds.

Something like 4000RPM = 55MPH in top - hard to say as the rev counter is a bit erratic, something else I need to sort out.

that matches mine 15t front 40 rear and same speed at same rpm, and yes my tacho is the same!!!!
going up 1 tooth makes the ride better and gear change not as fast, also enabled me to hold 30 mph without going up n down on the throttle as bike wanted to sit at around 40 mph

Suits what I want the bike for - road riding only, it would probably pull a 16t with no problems.
Guess the tacho problem is universal !!