Speedo issues

A

amajamark8

Registered
I had the transmission rebuilt in my '94 rebuilt, everything seems ok except for this. The speedo is off, 70mph reads about 83mph and the odometer is off about 1/10th of a mile per mile (over). I brought it back to the shop and they told me that the rebuilder accidentally put in the wrong speed sensor. He also said there is no speedo cable in the car and that the speedo and odometer work off the sensor. They kept the car, returned it to me a couple of days later with the "correct" speed sensor installed.

Needless to say, same problem!

Is there more to it than that, or do I still have the incorrect speed sensor installed?

Any ideas?

Thanks in advance.


amajamar
 
RE: Speedo issues

The sensor's probably just fine - but has the wrong gear. Did you change anything? If not, I think they changed the output shaft to one that has a different drive gear.

Here's a calculator...

http://www.sccoa.com/faq/speedgr.html

You can find these at many vendors...
http://www.transmissioncenter.net/FordSpeedo.jpg
AOD speedometer driven gears. AOD transmissions use either a 7 or 8 tooth drive gear. Once you change the gears in your differential or tire size, you need a new speedometer driven gear to recalibrate your speedometer. They're inexpensive and only take a few minutes to change. We have 18, 19, 20 and 21 tooth gears.
http://www.transmissioncenter.net/aode.htm

If you have 4.10s, you need this...
http://www.transmissioncenter.net/23T.jpg
23 tooth AODE / 4R70W speedometer driven gears. For use with 4.10 gears in 7 tooth manual and automatic transmissions.
http://www.transmissioncenter.net/aode.htm
 
RE: Speedo issues

Well, I never changed anything. How do I figure out which rear end ratio I have? Is the gear difficult to change?
 
RE: Speedo issues

[div class="dcquote"][strong]Quote[/strong]
Well, I never changed anything. How do I figure out which rear end ratio I have? Is the gear difficult to change?
[/div]

Let's hypothetically say the stock drive gear is 7 tooth and for whatever reason an 8 tooth drive gear was used. 8/7 = 1.14 and 1.14*70mph = 80mph. Sound like the problem?

From the calculator linked to in the previous post...
Speedometer Gear Teeth = (Drive Teeth * Rear End Ratio * 20.2) / Tire Height

Stock gears are 3.08:1
Stock tire height(225/60R16) is 26.63 inches.
Stock Speedometer Gear Teeth = (7*3.08*20.2)/26.63 = 16.35 = 16

Swap in the 8 tooth drive gear for the stock 7 tooth drive gear...
Speedometer Gear Teeth = (8*3.08*20.2)/26.63 = 18.69
Round up to 19.

Now you need a 19 tooth driven gear.

What will it do? Well, 16/19 = 0.842 and now 83mph*0.842 = 69.89mph!

As far as easy/hard to change depends. I needed to change mine when I installed 4.10s. Problem is my newly installed cat back dual exhaust prevented me from getting my big hands up beside the tranny to the sensor. So I had it done with some other work at a local shop. There is one bolt retaining the sensor in the tail section of the tranny. If you can get to it, remove the bolt and it pulls right out. The gear is retained with a spring clip on the end of the sensor shaft.

Good luck!
 
RE: Speedo issues

I think I'm getting it now. The drive gear is in the trans. So, that explains that even if they swapped speed sensors, the problem would remain!

Thanks for the math lesson!
 
RE: Speedo issues

The drive gear is on the output shaft in the tranny. The driven gear is on the end of the speedo sensor. I suspect they put the wrong drive gear in.
 
RE: Speedo issues

I suspect they put a different transmission in your car, not the original one and the new tranny had the different gear.
Roadie
95 Champagne Mark VIII
00 Ford F150 Supercab 4x4
86 Chevy Monte Carlo SS
 
RE: Speedo issues

Yeah, you're probably right. I ordered a 19 tooth gear yesterday. I'll see what happens once I install it.


amajamark8
 
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