We'll need to know what drive is presently in the tranny, if your speedo is reading high or low, and by how much.

Many years ago I was grabbing speedo drive gears from Turbo 400's and have a small selection of them.

This TCI calculator works fantastic,  and they have a list of all the plastic gears and # of teeth too.

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

So you need to reduce it 12%. 

If it's a 38 now, you should try about a 42 tooth. However, from the little I know about TH400 speedometer gears and sleeves, I think you will need to change sleeves to go to a bigger gear. I think the sleeve your gear fits into now is only made to go up to 38 or 39 teeth. I think the 40 and up, maybe even 39 and up use a different sleeve.

Found a decent video.

This is what I have,

35 teeth, so by my calculations, I need 39.77 .

A 40 tooth would be perfect.

I think after seeing the video, I will go with 39 teeth and call it a day !

No fluid loss, I will check the level later.

I took the advice of all above and I went off to see the guy who first rebuilt a transmission for me 30 years ago.

It took him seconds to pull out what I needed.

You have to wonder if brown is rare.

Like Clint, no brown but black and the proper case too !

Hard to count black teeth but I confirmed 40.

I traded him some parts and off I went.

Minutes to install and road test.

I am now just under 50 MPH on the speedometer when the GPS shows 50 MPH.

Problem solved !