There are 3 different speedometer drive gears used on the tail shaft in the muncies. The rear axle ratio will determine which one is used. That is probably why the numbers differ. If a speedo reducer adapter was used that may also reduce the number of different tags that were needed. Here is where I found the information,
http://home.comcast.net/~chevelleengineer/speedo.htm it could come in handy. Clint