Speedo off

[sdz]

My speedometer has been off since I purchased my 1991 earlier this summer. This weekend I installed a white gear (23 tooth), and it's closer, but still high. for example, at 65 MPG according to my GPS, my speedometer reads 75 or so.

As far as I know, there is not a speed gear with more than 23 teeth, so I must have to swap something else out.

I have no idea if the tranny is original, or the rearend.

What's the best way to get my speedo accurate? I would assume I need to open the 3rd member to inspect the ring & pinion. And as far as the tranny I have no idea of how to identify its ratios.

Thanks!

 

[Beasty306GT]

does it got rearend gears?? u need to change the speedo gear when doin gear swaps

 

[Mustang5L5]

WHat gears and transmission do you have?


On the tranny, if it's a stock t-5, you should have a green 8-tooth gear inside. If you pull the speedo cable out on the driver's side, look inside with a flashlight and you should see a green gear.

Out back, jack the two rear wheels up. Mark the tire and driveshaft and spin the wheels and count how many times the DS turns for 1 wheel revolution. If it turns just under 3 you have 2.73's, just under 4 is 3.73's. 3.5 times is 3.55's and etc.

Also, your speedo is a 140MPH speedo right?

 

[IH8BOWTIES]

As far as your gears in the rear end go, look for a small placard bolted to diff cover. You may have to clean it up a little or hold paper over it and rub with a pencil. The gear ratio is in the lower left corner of the placard and you're gear diameter will be to the right (7.5 or 8.8). If the gears are stock, this will be correct. If they've been changed, the placard may have been discarded. If that's the case, mustang 5L5's method is effective.

 

[Mustang5L5]

Not necessarily.

I have 3.73's in my car...but still put the tag back on that says 2.73's

 

[IH8BOWTIES]

Well, I said if it were stock. I've had to do a few you're way too. Either way is better than having to break open the rear end if you don't have to.
It almost sounds like you have an 85mph speedo with a 140mph face. That was the case when I got mine. Found a 140 speedo and the world is right again.

 

[sdz]

Good info. I will do some checking next weekend.

I do have the 140 MPH speedo.