I have the stock 16 inch rear wheel on my 81 GL 500 , how much more speed on the top end would you get with the 18 inch wheel? How about difference in gas mileage? And are the only ones available the reverse Comstar in silver? No black?
The metal finish CX wheels are (plain) "Comstars". The black ones are "reverse Comstars".
In Europe we got reverse Comstars on the standard model 1981. Last year for this model was 1982. The first years (1978 - 1980) that model had plain Comstars. 18" rear on all of them.
The Custom, Deluxe and GL models had reverse Comstars from the beginning (1979, 1979 and 1981). 16" rear on all.
The Euro and Turbo models had a different design, 18" rear with brake disc.
I don´t think you´ll gain much top speed with an 18-incher.
I own 1981 European spec standard models and have never been able to touch red in fifth. I had a Deluxe some years ago and with that one I was pretty close. But then again did the instruments on that one play their own games... So I´m not sure how fast it was.
What you´ll get with a 18-incher is marginally (but noticeable) lower revs (some few %). Maybe also marginally better mileage, but certainly not much.
I tried an 18-incher on one of the GL 500:S I owned many years ago. As a "fulldresser" it looked kind of strange with that thin rear wheel.