Ford Mustang VSS Background Information
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.
Here is the chart that shows the standard differential gear ratios and the speedometer gear combinations:
Axle ratio 6T drive gear 7T drive gear 8T drive gear
2.73 - 16 18
3.08 - 18 20
3.27 16 19 21
3.55 17 20 23
3.73 18 21 -
4.10 20 23 -
4.30 21 - -
4.56 23 - -
The standard Mustang tire size revolves at a rate of 809.9 revs per mile (Stock 145/45/17 tires calulated at http://www.1010tires.com/TireSizeCalculator.asp). 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 any online tire size calculator you like.
A word of warning about on-line tire calculators, however. Some calculators, such as Discount Tire Direct Tire Calculator will give you the tire revs per mile of a tire that is not mounted on a vehicle. When you mount a tire on a vehicle, the weight of the vehicle will cause it to compress a bit, which reduces the diameter of the tire a bit. I recommend this calculator, which seems to try to compensate for the effective diameter of a tire when loaded by the vehicle weight: 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 809.9 = 2648.373
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 2648.373 = 974.601264 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 = 7796.810112
Since the car electronics expects 8000 pulses per mile, the VSS signal error = Actual VSS rate/8000 = 7796.810112 / 8000 = 0.974601264 = 97.4601264% accurate for my car. Not too bad.
That means that, when my speedometer indicates I am going 60MPH, I am actually going 61.563638604105073272303902942609 MPH.
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 = 9787 pulses per mile, with an error = 1.2233752631578944 or 122.3% and when my speedometer indicated I was going 60MPH, I would actually be going 49MPH. If I chose to use a Speedcal, the input to the speedcal unit would be 9787 pulses per mile.
If I went to the 23 tooth VSS gear as recommended by D&D, my VSS signal would be = 8084.9 pulses per mile, with an error = 1.0106 or 101.1% and when my speedometer indicated I was going 60MPH, I would actually be going 59.37 MPH. Pretty close, so no speedcal needed. (However, we hear that the 23 tooth driven gears get chewed up after a few thousand miles, so a speed calibrator is recommended over a 23 tooth gear.)
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.)
Car Transmission Drive Gear Driven Gear Axle Ratio
94-95 All 5 Speed T5 8
96-98 Cobra T45 7 19 3.27
96-98 5 Speed Exc. Cobra T45 8 18 2.73
94-98 Automatic AOD 8
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:
Model Year Axle ID Code Ratio
1996 8(M*) 2.73
1996 5(E*) 3.27
1996 6(W*) 3.73
1996 Y(Z*) 3.08
1996 F(R*) 3.45
Source: 1996 Mustang Helms manual.
99-04 Mustang OSS background information
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.
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 transmission swaps.