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 unit from
Dallas Mustang can also be used between the VSS and the car electronics to compensate for the drivetrain changes. The electronic
Speedometer Recalibrator, part number M-4209ADPT-A from FRPP can also be used.
There is plenty of information on the internet about how to install a Speedcal or Recalibrator in your car, but there is not a lot of information that helps you to figure out how to set the Speedcal configuration switches. This page attempts to shed some light on how to get your Speedcal set up properly, especially for non-standard setups.
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. This results in a VSS signal that will generate 7854.88 pulses per mile, which is close to the nominal 8000 pulse per mile Ford spec.
When you change your tire size, differential gear ratio and/or speedometer gears, the resulting signal from the VSS will not provide 8000 pulses per mile, often by a significant amount. The changees to the VSS signal can be compensated for by inserting a Speedcal unit between the VSS on the transmission tailshaft and the PCM and instrument cluster. The Speedcal or Recalibrator reads the frequency of your VSS, and then scales the frequency of a VSS output signal to compnsate for the changes you have made in your driveline to bring the VSS signal back to very near 8000 pulses per mile. The switch settings in the speedcal or Recalibrator is what determines the frequency scaling that the Speedcal unit will perform on the VSS signal.
There are two models of Speedcals: Standard Range and Extended range. The switch settings are different between the two models, which creates some confusion on the internet. Dallas Mustang currently only sells the Extended Range Speedcals. They no longer sell Standard Range Speedcals, but there are many in cars and some used units for sale. There is only one model of Ford Speedometer Recalibrator.
The calculator above will tell you how to set your Speedcal switches for either model of Speedcal and for the Ford Recalibrator. In order to use this calculator, you will need to provide the stock VSS pulse per mile rate and the VSS pulse per mile rate of your new tire, differential gear and speedometer gear setup. The default value in the Stock VSS input field is the stock VSS pulse per mile rate for stock 1994-1998 Mustangs (and 99-04 OSS signals as well.)
In order to calculate your current VSS pulse per mile rate, you will probably want to use this Mustang VSS calculator:
Ford VSS CalculatorIf, for that calculator, you need the revs per mile of your tires, you can use this tire size calculator:
Discount Tire Direct Tire CalculatorYou can cut and paste the values from these calculators.
For those of you who want to understand how it works, the speedcal needs to know the ratio of the new VSS signal to the Stock VSS signal in order to do the compensation. The switches enter the scaling ratio in 10-bit fixed point binary formats of 1.9 for the Standard Range unit and 3.7 for the Extended Range unit. This means that the Standard Range unit can scale a VSS signal from 0.0x to just under 2.0x. The Extended Range unit can scale a VSS signal from 0.0x to just under 8.0x. The Extended Range unit gives a broader range, but lower precision than the Standard Range unit. (It is more likely you can get close to exactly 8000 pulses per mile with the Standard Range unit, assuing it scales far enough for your setup. The Ford recalibrator simply represents the scale factor as a binary number that represents the scale factor in integer percentage form.