I can't believe I've never seen this addressed, I'm surely not the first with this problem. I have a 67CS. The 400/2-speed has been replaced with what I believe is 79 350/3-speed. The speedo error is significant. Will changing the driven gear in the trans solve the problem? The gear housing is marked with 36,37,38 and 39.

 

kind of addressed today https://classicoldsmobile.com/forums...vs-speedo.html

 

Yeah that's what got me going. I spent about an hour on the speedo sites and other sites trying to identify what I have. I just thought it would be common enough that someone would have solved the same problem as mine. I have the engine and trans in the garage, but my car is in a shop for body work so I can't do the math to see what gear I need.

 

I needed a speedo correction when I discovered I was wearing it backwards.

 

THERE'S a mental image that I didn't need...

 

This might help.

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