The Stovebolt Site Forums The Parking Lot Big Bolts Speedo?

I have a 71 C-50 with a vaccumm operated 2 speed rearend. How does the speedo know what rearend gear the truck is in? Thanks.......................Flat67

There is a little transmission in the speedo cable that shifts at the same time.

Mr. Wrenchbender,

Thanks for your answer, I'll have to check out the "the little transmission" because I don't think it is working. I can shift the rearend but the rpm/speed doesn't change accordantly.....................Flat67

Flat 67,

The speedometer speed reduction unit should be atached to the back of the speedometer. It has a small vacuum diaphram that shifts the speedometer from high to low range to match the rear. All 2-speed rears have the same 1.36 differential, so all of the reduction units are the same. I'm betting yours has dry rot and that's why it no longer works.

Mike B

I have the same issue in my truck..

Hope I will just be able to fix it.

Mike B,

Thanks for the advice. Have you been successful in repairing the speedometer speed reduction unit? Sounds as if you have to be a watch repairman to fix one of these...............lol.........Flat67

Flat 67,

I haven't repaired one yet. I was able to find a NOS Diaphram a few years ago, but I'm afraid to install it for fear it's to old and will tear as soon as it's put to use. I'm thinking about sending it out to see if I can have repro's made new.

Mike B

Take a look at 70s emissions carburetors you will find a plethora of small vacuum diaphragms and linkage that with very little effort can be adapted to shift that little transmission.

Another little trick is to use the transmission from a truck that had an electric shift rear end and do away with the vacuum one.

The electric ones are usually remotely mounted with a speedo cable to and from where is the vacuum ones are direct mount to the back of the speedo.

BTW my 64 GMC has an electric shift speedo and a vacuum shift rear end and that is the way it came from the factory so you may even be able to adapt the whole system from a later truck. This would give you the electric shift transmission, the electric/vacuum switch and the needed cables

Thanks for all the tips guys...

HevyHauler,
Found the speedo reduction unit and it is like yours, electric shift speedo and vacuum shift rear end. I will start trouble shooting it, wish me luck........Flat67