With an 80T spur gear you should get pinions in the range 34 to 44 in order to cover all the likely combinations of tyre sizes and motor timing advance (static, not from the speedo) you might need to use on different tracks.
You'll usually find drivers very happy to loan you a pinion if you need one, so if this seems like a lot of pinions, get 34 to 38 to start with and see how you go from there. HTH
