Speedometer reading incorrectly after transmission change

[jdwyer25]

Okay so I've got a 1988 LX 5.0 Coupe and had a transmission issue that required me to swap transmissions. So the transmission that I put into the car is out a 1990 mustang 5.0. This was an AOD to AOD swap. So got the new transmission in and took it out for a spin and noticed that the speedometer was reading way off. So at like 55mph the speedometer is reading almost 70mph. So the question I have is will having an 85mph cluster in the 88 cause this problem since the cluster in the 90's is a 140mph. Or do you think this may be a difference in gearing between the 90 car and the 88. I need to take a look at my 88 and see what gears are in the rear end, but was assuming they were 2.73. The car the replacement transmission came out of was geared for 2.73. So I thought everything would swap and the speedo would be spot on.

I'm starting to wonder if maybe the 88 has like a 3.27 gear and I'll need to swap the transmission gear. I also have a 140mph cluster that I need to install from an 89 mustang as well. Really don't want to throw money at this till I get a better idea of what I might be looking at.

 

[Three50won]

jdwyer25, I did a little searching and the options were 2.73's, 3.08's, and 3.27's for those years. If your speedometer is off that much then the speedo gear in the trans you just installed is prolly for a 3.27. You'll have to swap the speedo gear which isn't too difficult. There are installation instructions and conversion charts everywhere. Try LateModelRestoration.com. And I think most companies give you detailed instructions when you buy the speedo gear anyway.

 

[Adams91LX]

My easiest method for determining rear gear ratio without disassembly.

For a non-posi rear:

Jack up one rear tire and rotate the tire exactly 2 revolutions while counting the number of driveshaft revolutions.
2 3/4 driveshaft turns = 2.73:1, 3 1/2 driveshaft revolutions = 3.55:1.

For a posi rear:

Jack up both rear tires and rotate one of the tires one revolution while counting the number of driveshaft revolutions.
2 3/4 driveshaft turns = 2.73:1, 3 1/2 driveshaft revolutions = 3.55:1.

You need 720 degrees of tire rotation for this method to work.

A Posi will spin both tires 360 x 2 = 720 degrees
Non-Posi will spin one tire two revolutions = 720 degrees

 

[jdwyer25]

Yeah I was looking at latemodel and yeah from what I can see yeah I was thinking I was dealing with 3.27 gears. I just need to jack it up and look at the diff tag. I'm guessing it's still there since the car is basically bone stock. It would be kind of nice to have the 3.27's as opposed to the 2.73 gears.

 

[Gearbanger 101]

Keep in mind, that the '90-'95 Mustang T5's utilize an 8-tooth speedometer drive gear, while earlier T5's utilize a 7-tooth speedomether drive gear. You'll need to swap out to a 7-tooth drive gear, or switch out the apoproate drvien gear to correct your issue.

The stock 7-tooth drive gear will be Yellow
The stock 8-tooth drive gear will be Green

Decyphering what rear ratio you've got out back will determine what colour gear you need on the driven (speed sensor) end.

Here's the Ford Racing speedometer gear chart for refference....

http://www.fordracingparts.com/download/charts/129.pdf

 

[jdwyer25]

I'm not dealing with a T5 transmission this is pertaining to an AOD transmission, but that is still good information to know for future reference.

 

[Gearbanger 101]

Crap...missed that part.

 

[jdwyer25]

I'm guessing that they are similar regardless. Since I'm using a 19 tooth gear and the only thing that changed was the transmission. So the guess is and won't know till I climb back under the car is that I need a 21 tooth with the 90 transmission as I'm pretty sure from research it looks like I'm dealing with 3.27 gears in the diff. It was great info though for sure either way.