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?