Removing TH400 tail section in the car??

[g356gear]

Hey guys,
I managed to pull the speedo cable and driven gear out of the side of the tranny last night and do a count of the drive gear teeth. I have an 18 drive gear and a 36 driven gear, which is the reason my speedo is so far out of whack. They were never changed when the tranny was swapped back in the early 70's to match the shorter 3.31 rear posi. From my calculations I have found I need a 47 tooth driven gear to match the 18 tooth drive gear, which nobody makes....max is a 45 tooth. This means I have to change out the drive gear inside the tail section of the tranny. Has anybody dropped a tail section off a TH400 while in the car or does the whole tranny need to come out? Thanks.

[GregWeld]

4 bolts and it's off...

There's a spring clip that holds the drive gear on.

BTW the 45 tooth would certainly be close enough... you're doing a lot of work for that little bit of accuracy difference.

[g356gear]

4 bolts and it's off...

There's a spring clip that holds the drive gear on.

BTW the 45 tooth would certainly be close enough... you're doing a lot of work for that little bit of accuracy difference.

That's exactly what my logic was too but I was told that the 45 tooth would still be about 10mph off.

[GregWeld]

Did you do the calculation - factoring in tire diameter etc? Do you know how to do that? If not -- here it is.... and even this "calculator" states to "round to nearest gear"....

Go here - scroll down -- you'll see the formula for doing this calculation.

You still might be right --- but I'm thinking that two teeth shouldn't make that big of a difference... but who knows?

[g356gear]

I did do the calculations based on a link found on a transmission site.

[g356gear]

I am going to try a 45 tooth and see where I am at. I really don't want to have to crack open the trans for 1-2 mph unless absolutely necessary.