Howdy,
I'm in dire need of a speedo gear that goes inside a SM420 GM transmission as well as the matching speedo gear adapter that threads into the trasmission and routes up to the speedometer for my 1957 GMC truck. It's a NAPCO 4x4 with 33" tires so also trying to find the right ratio to work with the tranny and drive train combo. Please let me know if you've got on or know where I could find on as mine is currently stripped out. Many thanks!
-Garrett-
sixfive0 Two08 Foureight84

Hi Garrett,

I've started using a bit more high tech solution. You can get dash mounted GPS speedometer (and does all kinds of other stuff) online for ~$50 dollars. Also they make GPS based in dash speedometers. And the one I use for stock in dash, and the most expensive one, is a GPS to speedo converter box.

With the latter, everything stays and looks the same but the box goes in the cab and then a short cable goes from the box to the speedo. Change tires, change settings...change wheels (and tires), change settings, change rear end, change settings.

I'm not sure what the rules are on posting links and specific products but the above mentioned is easy to search for.

Garrett,

Send Dan Conley an email, he rebuilt my tail shaft on my S10 T5 transmission, changing it from electronic to mechanical.

Pretty sure Dan will be able to fix you up. This is his website:

http://t56cablespeedometer.com/

email: cablespeedometer@yahoo.com

Dan can help you calculate the tire size vs gearing to determine the proper speedo gear that would best work. Will not be 100% accurate, but will get you close enough for rock and roll. There are different sets for S10 and/or GM, the TH350 has some that can be used for the S10 T5 also, as I recall.