I'm trying to calibrate my speedomter. Hopefully someone can help me. I have a 69 cutlass with a th400 trans. (plate reads 77 PC 10204 so I think it came out of a 77 pontiac?) Had an olds 12bolt 3.08 and the speedo was off about 10mph. Swapped to a chevy 12 bolt 3.73 posi so now the speedo is naturally even worse (15mph?) I saw some good info and fair prices at tciauto.com with charts of available gears and their respective colors and tooth counts. Naturally I figured I could just pull the cable and gear housing to see which color drive gear is in the trans in order to know which driven gear to use with my set up. Should be easy, but no. Turns out the driven gear is natural color but with 35 teeth. According to the chart orange driven gear would be 35 tooth and natural should be 36 tooth. No big deal since I expect to replace the driven gear with one that works with my set up (rear axle ratio and tire diameter) and the drive gear in the trans. So I look in the trans and I see the drive gear is also natural plastic. Unfortunately there is no mention of a natural color drive gear in the chart so I need to count the teeth somehow but I'm not sure how to go about doing this. Does anyone know how many teeth should be on a natural color drive gear in a th400? If I raise the rear wheels off the ground and put the trans in neutral will turning the wheels/driveshaft rotate the speedometer drive gear so I can attempt to get a tooth count?
Any tips/ info would be appreciated.

 

There are many different drive gears out there. I don't rememebr what was in mine (it was green) but I swapped to the correct 18 tooth for 1970. This is a pic from the service manual from 1970.
I hope this helps.

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