1966 C10 speedometer gear question.
 

I have a TH 350 with a 19 tooth nylon colored (yellowish) in my trans. I’m fixing the leak with a new bullet. My speedometer reads 50 when the truck is doing 55 according to GPS so it’s 5 mph off.

What color or amount of teeth gear do I need to correct this? I would like to correct this at the same time I’m fixing the leak?

I have 17” tires and a 373 rear end.

Thank you,

Greg

Re: 1966 C10 speedometer gear question.
 

I wish I could help you. I have the same problem but, my speedometer registers 90 when I'm only going 50-55. I have a TH400 and15" wheels. I know I need to change some gears in the trans, but I don't know which ones. I'm thinking about just going to a GPS speedometer. I hope you get it fixed.

Re: 1966 C10 speedometer gear question.
 

Re: 1966 C10 speedometer gear question.
 

I ordered a new bullet along with a 19 and a 18 tooth gear. I just read the old thread and the brown 18 tooth seems to give others a problem with the needle bouncing?

Worse comes to worse I’ll put the new 19 in and use my GPS which turns read anywhere I go over the posted speed.

Maybe the brown 18 uses a different drive gear?

My old 19 has a groove worn right where the old seal collar wore a spot that allows trans fluid to leak out.

#1 is getting this leak fixed.

Re: 1966 C10 speedometer gear question.
 

This is a pain in the @ss, isn't it? Here's a site to help figure it out. When going through the calculations, I did it 100 times to make sure the right size gear was selected. One thing I recall, if possible, try not to get gears on the max or minimum side because they're not as accurate. If memory serves right, it has something to do with the gear ratios between 1st, 2nd, and 3rd gear. If you want a little extra reading, click on this link. Good luck!

Re: 1966 C10 speedometer gear question.
 

I read this evening that if I’m running a 19 tooth driven gear and my speedometer is reading 50 and gps says 55 mph that I should add a gear with MORE teeth. I ordered a 22 tooth driven gear and hopefully this will get mine real close... 3 more teeth will get me 5-6mph.

Does this sound right?

The guy at the auto parts store said the same thing.

Thanks for all the replies.

Greg :chevy:

I’ll post my findings when I know for sure.

Re: 1966 C10 speedometer gear question.
 

If your speedometer is running slower than actual speed, I believe it's the opposite, the driven gear needs less teeth to speed it up. Less teeth increases the RPM on the speedometer cable. The opposite would be true if the drive gear was being replaced.