Do what I did and buy a GPS speedometer. More accurate than anything else. And they can be moved from car to car.

 

This doesn't really answer your question but may help you figure out how far it's actually off. I was having the same issue regarding the speedometer reading too high. I used my GPS to figure out how fast I was really going, which turned out to be 5 MPH slower than the speedometer reading. Purchased a driven gear with one more tooth and all is well.

 

Don't forget, there are 2 gears involved in driving the speedo.

The one on the cable that you can easily access.... and the one on the trans output shaft.

The problem is... the gear on the trans output shaft has the teeth rolled into the shaft itself.
You cannot change that gear without tearing down the trans and changing the entire output shaft.

The problem with this is there are 4 different output shafts with respect to speedo gear tooth counts for the C6.

6 tooth
7 tooth
8 tooth
9 tooth.

Trucks usually have the 7 tooth shaft, some may have the 6 tooth (very rare).

Passenger cars usually got the 8 tooth shaft.

If you have a 7 tooth truck shaft, no amount of changing the gear on the cable will get you an accurate speedo reading.

You "might" get close, depending on your rear end gear ratio and tire size,
but it has been my experience the best solution is to put the proper tooth count shaft in the trans.



Here's a calculator for you... scroll down for the Ford gears...

https://www.tciauto.com/speedometer-gear-calculator


Edit; looks like a 16 tooth gear will get you close...

Here's one...


https://www.summitracing.com/int/parts/tci-881000/overview/


Cheers!

 

Ever seen a 2wd C6 output shaft with no speedo gear on it? Got one in a rebuilt for a 71 Mustang. Unless the swapped in some other shaft. Runs and drives, but there is no drive for the speedo. Smooth shaft inside the cable hole. Checked with a bore scope. No drive gear.

 

These transmissions were also used in industrial applications.
It’s “possible” someone swapped in a shaft intended for something industrial that doesn’t need a speedometer.

OR that trans is actually from an industrial application and someone got it as a core and didn’t realize that.
Then rebuilt it and sent it out.
Can you see any casting numbers on the case anywhere?

Casting number example.
C6AP-XXXXXX

C= decade 60’s
6= year
A= full-size car ie Galaxie
P= Auto trans

“J” as the 3rd digit would indicate an industrial application.

You could for example, see a casting number
D7JP XXXXX
That would indicate 1977 industrial auto trans.