t_huth21

I have searched the web for some instruction or a video on how to change the speedometer DRIVE gear in a TH400 Trans. I've struck out. Has anybody made this change that can give me some insight/instruction? I'm making a pretty big rear gear ratio change and there is no driven gear size that will work with the drive gear size I currently have to get my speedometer back into line. Any help will be appreciated. Thanks!

4speed455

I did it on a th350. Should be similar for the 400.

Drop the driveline. Unbolt and remove the tail section which is four bolts. Remove the retainer clip. Remove the gear and replace. It literally took my less then 15 minutes.

joe_padavano

The only potential issue is that some transmissions use the clip and some have the gear pressed on. The correct puller has very long rods so you can bear on the end of the output shaft. You might have to make something. Install the new gear with a length of pipe.

t_huth21

Thank you guys for the responses. I was pretty sure that it wouldn't be a very tough job but I've never had to mess with the drive gear before, it's always good to get another opinion. Ok one other question on the topic. I've read and have seen in manuals in the past that GM used to use speedometer ratio adapters in some of the cars with lower (higher numerically) gears from the factory. They had different ratios, one of them being .6990. If I've done my math correctly I could up my driven gear a tooth and use the .6990 adapter and come out almost dead nuts. Are these adapters available anywhere anymore that anybody knows of?

BlackGold

I had a local shop build an adapter for me years ago. Ask a local transmission shop if they know of anyone who makes them. If not, I'm sure there's someone on the internet.

You can still find the original adapters at swap meets. But the chances of finding the exact ratio you need are slim.

Also, adapters are still available from GM, but only in a couple ratios.

droldsmorland

Can be accomplished with trans in the car. Had to purchase a small bearing backer clam shell from Graingers to adapt to a small puller(pressed on gear). Helps to apply minimal heat to the gear your removing. Gently heat the new gear after the shaft cools. It will almost slide on by hand. TH400.