I own 64 cutlass and replaced the 330 with 1977 350 motor. Recently swapped C-300 Auto trans and replaced it with TH 350 auto. After swap, my speedometer is 10-15 MPH off. My question is: can I swap the drive gear from the C300 and place it into the TH-350, or is there some other remedy? Modulator and speedometer cable have been replaced. Thanks for your help! Ken

 

The speedo drive and driven gears are unique to each model of transmission. They do not interchange. You need to get the correctly sized speedo gears for your TH350. Many trans vendors sell them. You'll first need to figure out the correct tooth count given your tire size and rear axle ratio.

 

Joe, thanks for your reply! Need to ask you one more thing. The only reason I swapped tranny's was due to C300 dropping a pint or more of fluid after sitting for 10 days or so. Are there any kits available that I can update the C300 so it does not leak tranny fluid? When finances become available, I plan on rebuilding the original 330 motor and would like to mate it to the original C300. Thanks! Ken

 

Your trans is actually a Super Turbine 300 (also called ST300). Oldsmobile used the marketing name Jetaway for this trans. I don't know what "update" you are looking for, but you can certainly rebuild the trans, or even just replace the seals if the trans otherwise works well. Fatsco is a good source of parts for this trans, as it is not common and most trans parts vendors don't carry parts for it.

 

Thanks for your help Joe, I will pass along information to my mechanic! Happy Holidays! Ken

 

Some useful info:

https://www.hotrod.com/articles/cali...-speedometers/

Also, plan on replacing both the inner and outer seals on the speedo gear housing on the trans when replacing the gear. This will help prevent oil leaks.