Speedometer Driven Gear question

[metalhead922]

Hello all,

I know this is a well traversed topic, but with my issue i cannot find any info on it so i figured id post a new thread.

it has been a while since i have posted on this forum, but i have come across a unique issue with my 66.. I recently swapped rear ends (complete drum to drum) in an effort to get my 66 back on the road after the original rear end blew.. it originally had 3.73 open diff, and in haste to get it running i bought a locally sourced good 3.07 open diff rear end. This was a pristine "economy" rear end from someone who had completely changed the drive train in their 66.

That being said, the install went well, no leaks, noises or anything else, but my speedometer was off by 10-15, so i followed numerous forum posts here and elsewhere, used online gear calculators, and all came up with the same solution, 18 tooth brown GM powerglide/th350 gear, so i ordered factory GM ones, and they actually came in a 2 pack. Upon opening the set i noticed that the brown gear was slightly smaller diameter than the original 22 tooth gear.. Read more forums and they all eluded to the fact that this is normal due to the tooth count.. i went to install the gear, had someone in the cab and i manually turned the gear by hand, they saw the needle move in the cluster, but upon installation, the gear does not make contact with the drive gear on the output shaft of the transmission, so.. no working speedometer.

Since they do not make a larger diameter driven gear in 18 tooth, the larger ones only go to 22, which i have.. am i left to change the drive gear on the output shaft?

[bhap]

If your speedometer works correctly and is off by a consistent amount, you can buy a ratio adapter to correct your speedometer. They are a small “box” that screw onto the transmission output for the speedo cable and then your cable screws on to that. You can go up or down in speed so if you read high, you can buy a less than 1:1 ratio and if low, a 1.x:1 adapter. I went through the driven gear diameter and teeth count nightmare and came to the conclusion I couldn’t buy the larger diameter gear in the tooth count I needed. The ratio adapter is simple and works great. Do the math for your actual vs indicated speed and then you can order an adapter that makes the correction. They are on *bay used or there are a couple of companies that sell new ones made to your specs. I forget where I bought mine (edit: Speedometer Cable USA), but can figure it out if you can’t find a seller. Good luck!

[PapaJbbq]

There's a multitude of driven gears available. According to this website, Powerglide and TH350 use same gears.
https://www.tciauto.com/speedometer-gear-calculator#350