Speedo Gear Selection

I purchased a T-5 from an '88 Mustang 2.3 and put it into my '88 T-Bird. I used my original vss and speedo gear (the stang trans didn't come with one). Now the speedo is off by about 8 mph.

(Edited out completely inaccurate paragraph, didn't want to cause any confusion for anyone reading this in the future!)

Thanks in advance for your help!

Update:

A 20 tooth VSS gear gives the speedo the correct mph and does not require you to change the gear on the tailshaft. Part # is C1DZ 17271 A

Explanation:

As it turns out the speedo gear on the actual tailshaft is different from '87 T-Bird to '88 Stang (both were 2.3 T-5 trans). You have two choices: (A) use the tailshaft speedo gear from your T-Bird or (B) use the tailshaft speedo from the Stang and order a 20 tooth VSS speedo gear.

Hope that is clear enough!

Hope this helps: http://mikewalsted.com/speedrpm.html

That's a little odd. You say the only thing you changed was the transmission, not the wheels, tires, or rear end gear ratio. The speedometer shouldn't have changed at all if you used the output shaft gear and VSS from the original T5.

What color are the output shaft gear and the gear on the VSS?

Sir the rule of thumb is one tooth for every 5 MPH change. So you did not mention weather your speedo was fast or slow?? So if its fast add one tooth if its slow subtract one. Sounds like you need 2 teeth to get you in the BALL PARK. Thank You

it's been a while but i think there are a couple of different gears that can be on the output shaft. it sounds like the transmission you put in has a different output shaft gear than the transmission you took out.

There is a 7 tooth and an 8 tooth gear on the output shaft. IIRC the Mustang use a 7 and the TCs use an 8.

The output gear on my 86 is 6 tooth (blk). And based on tire size and rear end gearing I think the T5 based 87-88 probably have a 6 tooth also. Someone should be able to verify that, is the output shaft gear black on the 87-88 T5?

Originally posted by Walsted:
Hope this helps: http://mikewalsted.com/speedrpm.html

I'm highly impressed! But I don't think it is going to be much help in this particular case. Because the problem is that the speedo gear on the tailshaft of the stang trans is different from the one on the TC.

Maybe it would be possible to add the Stang trans to the options list? IDK

Originally posted by Robert Camp:
if you used the output shaft gear and VSS from the original T5.

This is what I SHOULD have done, but I didn't even give a thought to changing it until I had it all back together and was giving it a test run.

I am hoping for a real simple fix that does not include removing the tailshaft housing. I believe that it ought to be as simple as just changing the gear on the VSS. I am just not sure which tooth gear to try on the VSS.

Originally posted by 5.0TurboCoupe1988:
it's been a while but i think there are a couple of different gears that can be on the output shaft. it sounds like the transmission you put in has a different output shaft gear than the transmission you took out.

Yes, there is. Unfortunatly I never give a thought to that before the installation .

According to this chart http://www.fordmuscle.com/archives/2000/...dex2.shtml the TC has a 6 tooth speeedo gear on the tailshaft and the 'stang has a 7 tooth speedo gear on the tailshaft.