We had a few races at San Jose Speedway in California back in the 70s. Supposedly a 3/8 mile asphalt high banked track. Our first race was a Saturday night exhibition between the heats and the main. We were all running Yamaha KT100's. Normally, in the KT100 Sprint class, I would use a 9 or 10 tooth driver, but at the Speedway I read a 13 tooth driver. It was kind of slow on the starts but once I got up to speed (and that's where you stayed for the whole race) I ended up winning by a huge margin. You don't want to go over your peak HP RPM. MPH is the key, keep a close eye on your RPM. If you can take one tooth off the axle, and still maintain the same (or close) RPM, that's always better. If you maintain the same RPM, take off one more tooth. Etc. etc. A chain oiler might be handy!