When I changed up from 29 inch diameter tires to 31's, I "swagged" the teeth count that my speedo gear should change and posted it on the board. Some one came back with a formula as to what gear it should be and I was not off. I just came to the size I needed differently! I seem to remember that the formula had the teeth count of the speedo gear. Do you know what size you have now that went with the last set of tires which ran accurately? I'll try and find that post.