Need some assistance please. I just swapped out my older TH350 with a newly rebuilt Th400.
everything installed great including the driveshaft, all cables, etc ...and no leaks

well...driving it home from my buddies house who had a lift, my ride who dropped me off to get the car, called him and he told me the speed we were traveling and my speedometer is 15 mph slower than actual speed.

I bought the transmission used from a 69 Firebird so I do not know what the driven gear is/ tooth count. Is this my issue ??

 

If you know your gear ratio and tire diameter, you can use this to calculate what drive/driven gears you need to correct your speedo.

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

 

It's not off by 15 MPH, it's off by a percentage and that changes the amount it is off as the speed increases.

You need to know what gear is in your car as well as the tire diameter and get the proper drive and driven gears for your transmission.

 

Thanks for the link and info. I have all of the data needed to come up with the calculation..but...
how do I calculate the current # of Drive teeth ? As mentioned this came out of a 69 firebird and I didn’t know at the time I needed to check it out. Do I have to pull it out and see what color it is...and possibly count the teeth ?

 

The gears are color coded, so pulling the driven gear and looking at both will get you started.
In 1970 they only listed an 18 tooth drive gear and everything is calculated off of that.
I have posted a picture on here a couple times of the different driven gears used with the different gear ratios and tires. You would need to look up the specs on the old tires to get the diameter and then the specs on your tires to choose the correct driven gear for your application.

 

T350 and T400 driveshafts do not interchange. Did you get a new driveshaft?

Something is not making sense. If you reused the same driveshaft then you have the same transmission. Also the kick down is not the same on a T350 as a T400 so how did you address that?

 

Yes I sourced a new driveshaft exact length of the factory option 442 with the turbo 400 fit like a charm

 

If you want to take apart the car only once, you could experiment with different gear counts. The idea is to get the driven gear tooth count as close to a whole number as possible, Obviously you can’t have gear with a partial tooth, if there isn’t a gear combination that is close to a whole number the factory used ratio adapters to slightly over or under drive the speedo cable to reach an acceptable calibration.

Once you have decided on a drive and driven gear count, order the gears and the correct driven housing. Make sure you order the drive gear clip if needed. Some early 400 have pressed on metal gears. You will need a small gear puller, and sometimes a little heat from a torch is needed to break things loose. The output shaft will have the hole for the speedo gear clip even if it had the metal pressed on gear

 

Here is the chart I was talking about.

 

Here’s what I have