Finally got my TH-350 3 speed trans rebuilt with the help of my genius buddy. Thankfully he had a Chevy version TH-350 laying around. We needed several parts from it. My core had alot of torn up metal inside. Apparently my core was rebuilt for race and destroy. It was mated to a big block before it imploded. It was missing springs and ball-bearings which I hear that people who want really hard shifts do. My stock rear end on my 66 Cutlass has a ratio of 3.08 and my tires went from 14" to 15". Will I need to change speedo gears? If so, does anyone have part a part number? Also, will my JetAway filler tube and dip stick transfer over? It appears to fit like a glove. My core was missing the filler tube and dipstick along with other stuff. The bell housing mounting tab on JetAway tube seems to match up nicely. Thanks for your help.

 

The size of the wheel has no effect on the speedo, only the OD of the tire and rear gear ratio does. How has that changed? That should guide you on any necessary speedo gear change. Most transmission web sites that sell speedo gears have calculators for this purpose.

The Jetaway tube/stick may not be the correct length / calibration for the TH350. Unless you can verify that they both measure the same fluid depth at Full & Empty, I would advise sticking w/ a tube/stick for the TH350.

 

As mentioned by JohnnyBs68S, the key will be rear tire overall diameter.

Post #5 from Joe Padavano, in thread below.

https://classicoldsmobile.com/forums...cutlass-22508/

https://classicoldsmobile.com/forums...adapter-39695/

Hopefully those threads help some.

 

Thanks for your help. Tire size.com was a good place to check. The old Sears tires on the old steel rims were FR78-14 which converts to 205/75R14. I put BF Goodrich P235/60R15 on my new 15” Vintique rims. The speedo vs actual speed calculator said the difference was negligible. Less than 1 mph when going 65 mph. Seeing I now have a 74 TH350 trans (that came from a Buick plant) vs my stock 1966 Jetaway trans, should I compare the color of the speedo gear sets and if they are different and swap in my original speedo gears?

 

I'm not 100% sure, but don't assume the Jetaway and TH350 speedo gears are interchangeable. If they are, then it should be safe to assume the color coding is the same. Don't forget to check the color of the drive gear that is attached to the output shaft as it affects the overall speedo gear ratio too.

 

The speedo gear on the BOP TH350 is interchangeable with the Jetaway. The speedo gear on the Chevy TH350 is different than the BOP.