Speedometer Reading Higher Than Actual Speed

[Jamchin]

Hello All.


I have just purchased a 2001 SVT Cobra. The previous owner said that the transmission had been either replaced with another TR-3650 or taken out and repaired. He did not know if a repair occured and is not sure what specifically, or a complete swap. I know for a fact that the speedometer is reading atleast 10km/h faster than my actual speed (I tested by driving side by side with my buddy). The final drive gear ratio has not been changed and is stock (3.27). I have done some research on this and I am not sure if the wrong speedometer gear that drives the VSS was installed (the driven little gear). How does the vehicle speed sensor (VSS) work for my car? It is mechanically driven by this gear? Or is it all magnetic/electronically controlled? Some forums say that my car is mechanical, and you just need to replace the gear with a (smaller or bigger not sure yet). Some say you have to tune it or something, which costs big bucks.

Someone on this forum told me:
Starting in 1999, Mustangs went from using a VSS (Vehicle Speed Sensor) to an OSS (Output Shaft Sensor.) The gearing of the VSS in the older cars is designed to provide the PCM, Cruise Control and Cruise Control system with an 8000 pulse per mile signal. In the newer cars, the OSS sensor gives a fixed number of pulses per output shaft (12 for stock manual Mustang transmissions.) The PCM then calculates the speed based on that signal and the tire revolutions per mile and axle ratio values stored in the tune. (The number of holes (or pulses) on the output shaft is also programmable, but is always 12 for a manual tranny.) The PCM then sends the calculated speed data to the instrument cluster and cruise control module via the CAN bus.


The only ways to tweak the vehicle speed in your car is by tweaking it in the tune (done by your tuner or by a Ford dealer) or by using a recalibrator such as a Dallas Mustang Speedcal.



Where exactly is the VSS/OSS and this gear (if it exists) located?
Can someone please educate me and tell me what's going on? Why would my speedometer read much faster? Could the output shaft been changed? It's driving me nuts because my mileage is falsely being recorded giving me more mileage than I actaully drive!


Thanks!

 

[soccerman002]

What size wheels/tires are you running?

My understanding is that you'll need either a tuner, calibration tool, or have your PCM reflashed at a dealer.

 

[Jamchin]

The tires and wheels are not the issue, because the overall diameter of my wheel is almost identical to OEM rims + tires. OEM tires are 245/45/17, my current tires are 285/35/18. So OEM wheel + tire diameter is 25.68 inches and current wheel + tire diameter is 25.85 inches - pretty much the same.

My question is, why is it reading so much higher if no components were changed?

And if something was changed, what could it be?
-Output shaft? (maybe more splines on it or something?)
-VSS/OSS malfunction or changed?

How much do you think it would cost to get this re-flashed or w/e? I would assume kind of a lot for such a simple thing...I wish it was like the older stangs, where you could replace and play around with the driven gear :S.

Thanks.

 

[JCP281]

Not doubting you on the gear ratio, but have you physically jacked the car up and rotated a rear wheel to see how many times the driveshaft rotates for 1 complete wheel rotation?

 

[shurur]

PM mwolson or ask him to chime in.


Edit:
I see mwolson answered you in the sn95 forum.

"The only ways to tweak the vehicle speed in your car is by tweaking it in the tune (done by your tuner or by a Ford dealer) or by using a recalibrator such as a Dallas Mustang Speedcal."


If you get a speedcal, he can help you with what to set it to. Dallas also has a table you can use to get in the ball park.

The speedcal mounts under the shifter bezel. It connects between the vss and the computer/speedometer.

http://www.dallasmustang.com/news/2014/01/06/speedcal-faq/

Connect the speedcal power to traction control power!!!!

The 99-01 cig lighter is constant on...so speedcal will drain your battery over a short time!!!

Do this instead:

http://www.miracerros.com/mustang/axle.html


99-01 cobra came with 3.27. I bet you have 3.73 or 4.10 diffy installed. Check it by doing what the above poster said.

 

[Jamchin]

Hey shurur.

I am very sure that my diff is stock. I have already been in stangs both old and new with differentials being changed to 3.73 and even 4.10...mine is stock, it takes forever to get through my gears haha.
Which still leads me to the question: How did my speedometer begin reading like this? Like it's ridiculous...maybe the output shaft was changed or something?

What could it be...

 

[shurur]

Out of my expertise.

The only thing I can relate to, is the new gem not being programmed in my ranger....speed was crazy and miles clicked by.

Not even sure the gem is involved on the mustang..

PM mwolson.

Maybe get a code reader to see if you're throwing any codes..but not sure anything would show with the mustang.

And make sure everything is connected.

 

[mwolson]

Hey shurur.

I am very sure that my diff is stock. I have already been in stangs both old and new with differentials being changed to 3.73 and even 4.10...mine is stock, it takes forever to get through my gears haha.
Which still leads me to the question: How did my speedometer begin reading like this? Like it's ridiculous...maybe the output shaft was changed or something?

What could it be...

All of the Mustangs with manual transmissions and OSSes have 12 holes per rev of the output shaft. The PCM reads the OSS signal, calculates the speed and sends that data in digital form out via the CAN bus. The only things that can make the data wrong is different tires (typically not a large difference), a different axle ratio (definitely noticeable), or different values in the tune (Number of OSS Holes, Tire Revs Per Mile, and Axle Ratio.)

The easiest thing to do is to actually check the axle ratio. The next easiest thing to do is get a dealer to tell you the settings in the tune.

If you don't want to do either, you can install a Dallas Mustang Speedcal or equivalent to correct the OSS signal using brute force.