mustang transmission swap - Ford Mustang Forum
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post #1 of 5 (permalink) Old 04-11-2014 Thread Starter
gtmustangt's Avatar
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mustang transmission swap

I have a 1996 Mustang GT has a 1997 Cobra transmission and the speedometer is reading way too fast help

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post #2 of 5 (permalink) Old 04-11-2014
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Welcome to the site!! I moved your post for a better response.

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post #3 of 5 (permalink) Old 04-11-2014
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With any heavy modification, I would visit a tuner shop. They should be able to properly calibrate the gearing issue on your speedo and you will pick up several additional RWHP in the process with the custom tune. I had to buy the SCT tuner and then pay an additional $300 for the shops time to Dyno and tune. Total time was several hours since they do several pulls and allow your car to cool in between them.

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post #4 of 5 (permalink) Old 04-11-2014
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The post does not state HOW the speedometer is off. Is it a set amount or a percentage?

The 96-98 model year uses a VSS sensor. The 99+ uses an OSS sensor. How the sensor outputs are used differently between the model years. The output of the VSS sensor goes to the cluster AND the PCM.

The OSS sensor output goes only to the PCM. The PCM then "tells" the speedometer what speed to show.

What this means is that a PCM tune can't fix an incorrect speedometer for a 96-98. The tune will indeed all the PCM to have the correct speed. But the speedometer will still be showing the same thing.

The accepted way to correct the speedometer after a gear change on the 98 and older Mustangs is by changing the tail shaft gear that drives the VSS sensor.

How to determine if your speedometer is the problem. Use an ODB2 scanner and monitor the speed as reported by the PCM. Compare to the speedometer display.

One other thing. The VSS and OSS sensor are NOT compatible to each other. Using the wrong sensor will also make the speedometer incorrect. Usually the error is HUGE.

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post #5 of 5 (permalink) Old 04-11-2014
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If I read this correctly, the car is a 96 GT and the speedometer was working correctly until the 97 COBRA transmission was installed.

The COBRA Transmission has a 7-tooth gear on the output shaft and the GT came with an 8-tooth internal speedometer gear. This will make the speedometer run fast.

What color is the gear on the tip of the VSS sending unit on the transmission?
Is it the same VSS and gear that was on it before the swap?

Or, if you know the differential ratio we can go to the charts and figure out what small gear mates with the large 7-t gear in your COBRA T-45.

Stan -
1965 5.0 Coupe /MAF/EFI/Astro A-5 T-5/B303 Roller Cam

Mustang Manual Transmission Specialist
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