Rear tire diameter...

[BigWeld]

To those of you with Hawks, Tt250s, etc with the 18" rear rim, my question to you all is this: what is the outside diameter of your TIRE.

I own a Brozz with the 19/17 highway combo, and in order to decide on a new rear sprocket, I need to make an educated decision on the ratio I want. Trouble is, outer diameter plays a huge role on output rpm. Need an example? If you compared the outside circumstances of a 17" tire and an 18" tire, the distance is different. At rpm, the taller rear tire makes fewer revolutions to achieve the same distance as a smaller tire at a higher rpm. Maybe the difference is minuscule, maybe it's not.....but I'd like to calculate the difference and adjust my choice of rear sprocket accordingly.

Thanks in advance!