It depends on the type of rear shock absorber you have and type of tyre tread you fancy,mate.
I used a 130/80-18 Michelin T63 on my R80g/s once and although it fitted okay with the original rear shock,the side knobs of the tyre rubbed against the spring of the Spax shock I later fitted.
The T63 was quite chunky though.....a later Sirac tyre had more clearance.
To further confuse the issue,some tyre brands measure across the actual tread while others just measure across the sidewalls.
So you`ll see how different styles and brands of tyres can give a 'yay' or a 'nay' answer to you.
Also check your rear rim size...some had 2.15 (or was it 2.25) rims while others had 2.5 ones.
I've got 2 brand new F/R sets here...Cost £120 a pair...
They are still available, as Metzler have done a short manufacturing run every year..
I've got another set on order for when that happens...
PS ....
Love how the works going with the 2 airhead rebuild threads.
Looking forward to seeing both Maverick and Bill [jaffa cake]H's
bikes at next WeeKend..