Scooterotica

acording to tyre guide in stickys book 100/90 givs 0% gearing change

Its the circumference you will need to measure. Judging by these figures, 444mm and 448mm diameter its only 13mm difference in total. This equates to approx 0.1 MPH per 1000 rpm which will be virtually undetectable, by me anyway.
Also if you going to fit them to a wide rim this is more likely to decrease the diameter surely ?
I may be wrong but I saw none at all.

the 100/90 is aproximately the width of 4.00/10 and the profile of a 3.50/10
you can get 100/80/10 tyres which will lower your gearing about 4/5%
another thing ive found is that different tyre makes vary quite a bit in size