Recalibrating speedometer by changing the drive gear on a Th-400

[SlickGTP]

I did this with my TH-400, but most automatic transmissions will be similar. I also went the long way around and replaced the drive gear. Some will be able to just replace the driven gear which is much simpler as it is one bolt for the retainer on the Speedometer Pinion.

1) Jack up the back of the truck and support with jack stands so that most of the fluid is towards the front of the transmission.

2) Remove the driveshaft.

This is what you'll be looking at:

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I have a ratio adapter from the factory. With this fix I can correct the speedo and get rid of the adapter.

3) Remove the tail housing. To do this I had to use a piece of wood to spread the load across the transmission pan and a jack to lift the transmission so I could remove the rear mount and clear the crossmember for the lower two bolts.

Result:

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4) Remove the Speedometer Pinion. There's one bolt that holds the retainer and then the pinion pulls straight out.

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You'll notice the governor gear behind the speedometer drive gear. During all this you want to prevent any foreign objects from getting into the transmission.

5) Remove the drive gear. This is actually a lot easier than I had first imagined. If your output shaft has an o-ring, remove it first. Then press down on the metal clip and slide the gear backwards. Be careful as the clip will fall as you slide the gear off.

New gear vs old gear...

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Off to the left you can see the speedometer pinion with the driven gear.

6) Install the new drive gear. I found it helpful to put the clip in place and then rock it backwards so that the gear would more easily slide over it and then slide the gear over it allowing the clip to locate in it's place on the output shaft until everything clicked in place.

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It's a little fuzzy, but you can see the clip at the bottom of the shaft at the edge of the gear.

7) Replace the o-ring and seal on the speedometer pinion and reinstall. For the OEM pinions there is only one way for them to go on so make sure that it's rotated so that the retainer fits into it's indents on the pinion. Bolt in the retainer and call it good.

8) Clean off the old gasket and replace the seal and bushing in the tailhousing. Unfortunately I don't have pictures of what I did here. For the Seal there's a small hole that you can put a punch and hit the seal from behind. Hell of a lot easier than any other method. Sadly I didn't realize this until fighting with it from the outside for awhile.

The bushing I ending up carefully using a chisel to cave it in and down until it dropped out. Then I used a piece of flat steel that fit over the entire bushing to help drive it in as I didn't have a bushing driver. Just go slow and it'll go in just fine.


That's all... Lube up the rear seal and reinstall the driveshaft, drop the truck back on the ground, take her for a spin and verify that the speedometer reads correctly!

 

[HotRodPC]

Those ratio adapters are USUALLY not worth a damn and wear out very quickly and quit working.

 

[SlickGTP]

The one I pulled off is a heavy duty sucker... Would probably last longer than I will, but then again it was probably put on by the factory in '73 when they knew how to build things to last.

Nothing like the one I bought off of ebay only to have it fall apart in under 5 miles.

 

[Swims350]

in order for it to work without changing the gear on the trans output shaft you would have to find out how many teeth it has, what your gear ratio is, and what 2 gears match together or various combinations of the 2 gears to make sure you could even get a gear for the small housing to match up with the one in trans and have it work together and be correct.

So for anybody thinking Oh I can just change the housing gear and be done, well maybe, but maybe not. Find out some info first then ask someone who sells the gears or search around online to find what gear combos you can use.

IF NOT then just swap the shaft mounted gear, it's not that bad, just sometimes the clips are bad for breaking, but are sold where you buy gears from online. th350's have the clip, no sure if 400's or others do or not.

 

[HotRodPC]

The one I pulled off is a heavy duty sucker... Would probably last longer than I will, but then again it was probably put on by the factory in '73 when they knew how to build things to last.

Nothing like the one I bought off of ebay only to have it fall apart in under 5 miles.

My dad once did a gear and tire swap so he paid $80 for a speedo calibration. The ratio adapter broke a week later and then he had no speedo at all. Took it back, again lasted a week. He went about 2 weeks when he had time to get in, they put a different type of ratio adapter on. It lasted 3 weeks and took it back and was told he only had a 30 day warranty, so he'd have to pay $80 again if they wanted it fixed. Funny thing was, there were 2 other customers there bitching for broken ratio adapters and were asking for refunds before their 30 days was up. There are certainly some bad ones out there.

 

[HotRodPC]

in order for it to work without changing the gear on the trans output shaft you would have to find out how many teeth it has, what your gear ratio is, and what 2 gears match together or various combinations of the 2 gears to make sure you could even get a gear for the small housing to match up with the one in trans and have it work together and be correct.

So for anybody thinking Oh I can just change the housing gear and be done, well maybe, but maybe not. Find out some info first then ask someone who sells the gears or search around online to find what gear combos you can use.

IF NOT then just swap the shaft mounted gear, it's not that bad, just sometimes the clips are bad for breaking, but are sold where you buy gears from online. th350's have the clip, no sure if 400's or others do or not.

Usually the difference you're talking about is if you're going from like 3.08's way down to like 4.10's or 4.56's, but then agian, not always. Sometimes you might just change tires by 4 inches and it require a different gear combo. In MOST cases, if you have the big yellow or funky off color big gear to the speedo cable, then there is an option for the output shaft.

Th400 used both clip type and pressed on speed gear where you'd have to use a puller to get it off. Clip type is most common. And even if you have the pressed on type, the hole is still there to convert to the clip type.

 

[SlickGTP]

Usually the difference you're talking about is if you're going from like 3.08's way down to like 4.10's or 4.56's, but then agian, not always. Sometimes you might just change tires by 4 inches and it require a different gear combo. In MOST cases, if you have the big yellow or funky off color big gear to the speedo cable, then there is an option for the output shaft.

Th400 used both clip type and pressed on speed gear where you'd have to use a puller to get it off. Clip type is most common. And even if you have the pressed on type, the hole is still there to convert to the clip type.

Yeah, for my tire height/gearing ratio combo I needed a 2:1 ratio between driven/drive gears... With a 15 tooth drive gear, I found they didn't make a 30 tooth driven gear, so I had to change the drive gear. Lucked had it that I had a 42 tooth driven gear and I found a 21 tooth drive gear on eBay.


That's handy to know... I was wondering if a shaft with a press on gear would have the hole for the clip or not.