Identification and function of this part!

[syncrov69]

Anybody know what is the function of this part placed between tranny and speedo cable on my 1971 C30 TH400. Both end (male and female fitting) look to be the same size, is it possible to cancel this part and hook speedo cable directly on tranny speedo fitting. Thanks

[Wrenchbender Ret]

That is called a ratio adapter. It is used to calibrate the speedo. There are little gears in it that can be changed to get the speedo acurate. You can remove it but it will change the speedo reading. Sometimes it makes it more acurate without it, as so many changes have usually been made since the truck was new. Match the odometer reading with the mile markers on the freeway to check the accuracy. If it turns hard you can take it apart & grease it. If the gears are stripped you may have to get it rebuilt to get the speedo accurate.

[syncrov69]

Is it possible to buy new one? Where it's possible to buy part to rebuild it or where you ship it for a rebuild? Is it C30 specific part to compensate for big tire size compare to a C10 tire size?

[mrein3]

Originally Posted by syncrov69

Is it possible to buy new one? Where it's possible to buy part to rebuild it or where you ship it for a rebuild? Is it C30 specific part to compensate for big tire size compare to a C10 tire size?

I purchased a used th350/NP205 out of a Blazer. The Blazer had 3.07 axels. It had one of those on the speedo output of the transfer case. So no, it is not specific to C30s.

As near as I can tell, NP205 speedo outputs are geared to make a stock speedo work with 3.73 gears and 28" (stock) tires. I come to this conclusion because the 205 I removed (manual trans version) worked equally as well as my th350 version of the transfer case without that adapter in there.

Taller tires would have the same effect as changing to a numerically lower gear ratio in the axel. So yes, it may have been installed to compensate for taller tires.

Chevrolet used this transfer case well into the 80s so if you're looking for a used replacement, check under 4x4s in the bone yards. I've never seen or heard of one of these on a 2wd truck before.

I'm hanging on to my adapter incase I deside to run bigger tires in the future.

[steves1967]

I have one of those adapters on my 71 c-20. Factory original. Mine was leaking fluid- local tranny shop was able to replace the gasket in it and now all is well again.

[dwcsr]

Try these guys
http://www.ddspeedometer.com/products.html#RATIOADAPT