Race bike drive ratios, sprockets anybody understand them?

[Jamie S]

Posting this for a friend , any help much appreciated.

I've just fitted a 17" rear wheel into my RGV Rotax powered supermono which replaced the skinny 18" it came with, the original rear wheel had a 40T sprocket and the gearbox 17T. Now to get perfect chain alignment I will have to have a offset gearbox sprocket made,but before I do that I'd like some advice. I can't go much smaller than a 17T front because of the swingarm but can go up to a 20T, what size rear am I looking for? I know suzuki do a 45T and a 43T rears off the shelf,

17T 40T

18T ?

19T ?

20T ?

Because of the tyre profiles the rolling radius of the 17" and the 18" are practically the same. According to the Rotax manual the Primary ratio is 32:76 and 5th gear is 21:23=0.913 with an option of 21:22=0.955 (I don't know what I've got because I bought the bike with no history)

Has anybody got any idea if these ratios are any good for racing?

thanks in advance