I'd want the mint 32 toother in there just cuz how bad the 31 toother is but it'd show the MPH a hair low than actual MPH (might cost you a ticket, might not). The archives has a chart & someone posted a link for a comprehensive chart but I dont have that info. EDIT measure your actual tire dia to get it as dead on as possible (might change what tooth count you need). MORE EDIT the chart someone posted had a MPH/RPM chart that was very helpful & there was something about an Excel worksheet. EVEN MORE EDIT the Excel worksheet in the 2 links are right there on the same page, right in front of me, I missed it.