Accutach Company

 

 

1994-1998 Mustang speedometers, odometers and PCMs expect a Vehicle Speed Sensor signal with a rate of 8000 pulses per mile. Changes in tire diameter and/or differential gear ratio will cause this signal to become inaccurate. The speedometer drive gear on the transmission tail shaft and the speedometer driven gear on the VSS sensor can be changed to try to compensate for these changes. An electronic Speedcal-style unit can also be used between the VSS and the car electronics to compensate for the drivetrain changes.

D&D Performance sells the gears you can use to compensate for the changes. Their Speedometer Gear Chart can be found here: D&D Speedometer Gear Chart. 

Here is the chart that shows the standard differential gear ratios and the speedometer gear combinations:

The standard Mustang tire size revolves at a rate of 815 revs per mile. The standard differential gear ratio for my 96 Mustang Cobra is 3.27. My stock T45 5-speed transmission and my current D&D T56 6-speed transmission each have a 7 tooth speedometer drive gear. My stock VSS has a 19 tooth driven gear on it.

The calculator above can help you plan or understand your drivetrain setup. By putting in different tire revs per mile, you can see the impact of different tire sizes. By entering different differential gear ratios and speedometer gear tooth counts you can see the impact of them, so you can check out the impact of any combination. It is possible to enter non-standard numbers for any of these items so be careful that you enter the right data for the setup you want to check.    If you need to find out the tire revs per mile, you can use this calculator: http://www.1010tires.com/TireSizeCalculator.asp

Here is an explanation of the calculations done by the calculator above, along with two examples:

• The driveshaft revolves at a rate = Differential Gear Ratio X revs per mile = 3.27 X 815 = 2665.05 Revs/Mile for my car
• The speedometer gear ratio = drive gear / driven gear = 7/19 = 0.368  for my car
• The VSS shaft revolves at a rate = speedometer gear ratio X driveshaft revs per mile = 0.368 X 2665.05 = 981.860  revs per mile for my car
• There are 8 teeth on the VSS reluctor, with one pulse per reluctor tooth, so the VSS signal = 8 X VSS revs per mile = 7854.88

Since the car electronics expects 8000 pulses per mile, the VSS signal error = Actual VSS rate/8000 = 7995.15 / 8000 = 0.981861 = 98.1861% accurate for my car. Not too bad.

That means that, when my speedometer indicates I am going 60MPH, I am actually going 61.10847558475183MPH.

If I were to change my differential gear ratio from the stock 3.27 to 4.10 without changing the driven gear on my VSS, my VSS signal would be = 10024.5 pulses per mile, with an error = 1.2530625 or 125.3% and when my speedometer indicated I was going 60MPH, I would actually be going 48MPH. If I chose to use a Speedcal, the input to the speedcal unit would be 10024.5 pulses per mile.

If I went to the 20 tooth VSS gear as recommended by D&D, my VSS signal would be = 8019.6 pulses per mile, with an error = 1.00245 or 100.2% and when my speedometer indicated I was going 60MPH, I would actually be going 59.85MPH. Pretty close, so no speedcal needed.

This calculator can help you to decide if you want to use a speedcal or speedometer gears or both.

 

 Here are some common stock configurations for your reference: (Please contact us if you think any of this is wrong or want to add stock configuration information.)

 

 

Here are some common stock configurations for your reference: (Please contact us if you think any of this is wrong or want to add stock configuration information.)

 

Sources: www.corral.net, http://www.allfordmustangs.com/Detailed/349.shtml, http://www.latemodelrestoration.com/page/Mustang-Speedometer-Gear-Application-Guide

 

Available Mustang Axle Gear Ratios & ID Codes:

*Traction Lok

 

Source: 1996 Mustang Helms manual.

 

 

Unlike the 94-98 Mustangs, the 99-04 Mustangs transmissions do not have a VSS, they have an OSS (Output Shaft Sensor).  The PCM reads the frequency of the OSS signal, and calculates the vehicle speed based on the number of teeth in the reluctor wheel on the tailshaft, the axle gear ratio and the tire revs per mile.  For example, there are 12 teeth on the T56/T3650 OSS reluctor wheel and 6 teeth on the 4RW70W OSS reluctor wheel.  This leads to a significant difference in speed signal timing between the 94-98 and 99 and up generations of transmissions and cars.

 

Source: http://www.modularfords.com/2008/08/08/installing-a-4r70w-into-a-03-04-mustang-cobra/

 

How to translate speed signals in transmission swaps across VSS/OSS generations

 

Checkout our write-up on how to set up your speed signal in cross generation trnsmission swaps.