Speedometer Cable

[hsdropout]

I have installed a TH350 transmission and want to use the stock speedometer and cable..........the fit is fine.
My question is, will this give the correct speedometer reading or do I need different gear at tranny?
I thought I had seen something on this before but can't find with "Search" button.
Thank you.

[Coach529]

Subscribed.....

[mr48chev]

Your gear at the transmission will have to reflect the rear tire size and gear ratio in the rear axle.

This page is one that a lot of us refer to to figure out what drive and driven gears we need in the transmission to correct the speedometer. http://www.transmissioncenter.net/sp...n_______va.htm
You can usually get the gears locally at the local GM dealer or a good Transmission shop.

You can most likely come pretty close by figuring it out ahead of time but what I usually do now is to drive down a straight stretch of road at highway speed and check the speedometer against a GPS. Then figure the percentage it is off and which way it is off and by checking what gears are in the trans i figure out what gears I need to change to.
We usually have, or have relative with a hand held gps that will read speed in mph. That beats the daylights out of running down the road with someone following you to check their speedometer against yours.

There are also little gear boxes that hook to the speedometer drive on the trans and the cable hooks to them, you swap gears inside them to correct the ratio. A lot of 4x4 guys use them to correct the speedometer when going to different tires and many semi truck shops have them. you would need to know the percentage of correction you needed though as they have to be set to correct your speedometer.

[OrrieG]

I think those gear reducers were standard on some late 70's + 4wd trucks. There was a discussion in the 4wd section a while ago about them and how to get the right speedo reading. A forum search there should turn it up. I've got a th350 in mine but never had a working speedo. I just went with the flow.

[mr48chev]

Here is another chart that might be a little or whole lot easier to follow.

http://www.tciauto.com/tc/speedometer-gear-calculator/

[hsdropout]

Excellent! thank you Mr 48............good stuff.
I think you are right Orrie, my tranny has the little reducer box....must have been standard equipment.
CHP tells me I need a working odometer among other things to get truck legal again.
Hoping to keep original black and yeller plates.
Thanks again..............

[_Ogre]

a working odometer you have. that's what they want to see.
get your truck on the road and do like mr48 says
figure out the percentage you're high or low and go from there.

mine was way off, i did a 100 mile test against the mile markers on the freeway to figure out how off i was
it was %14 off, now truk is within 1%

[hsdropout]

Ogre, 14% off.................so when you were crusin along at 86 MPH you were actually doing 100? or was it the other way?
I am curious to see what mine does.........everything stock except motor/tranny.

[_Ogre]

cruising at 86 it read 100
now it is fixed 100 means 100



if your running a stock rearend, at 65 your motor will sound like it wants to explode