Tom, I fooled around for quite a while trying to get the correct gear for my speedometer.
My issue was the gear holder that fits into the transmission - there are two of them (up to 39 tooth and 40 tooth and up) and I needed the one that I didn't have.
Once I got the correct holder I found the gear which is similar to this;
http://rover.ebay.com/rover/0/e11050...AMOTORS%3A1123
IIRC, the gears for these transmissions are the same as the gears for the TH400.
Greg.