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 was also an old version of the M-4209ADPT-A that used 8 binary on-off switches to set the compensation factor. The new version uses three 0-9 rotary switches to set the compensation factor.
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 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 changes 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 compensate 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. Dallas Mustang no longer sells Standard Range Speedcals, but there are many in cars and you may find some used units for sale.
There are two models of the Ford recalibrator, part number M-4209ADPT-A: the old model and the new model. The old model has an 8-bit binary switch bank while the new model has a 3 digit 0-9 rotary switch bank used to set the correction factor. FRPP no longer sells the old model Recalibrator, but there are many in cars and you may find some used units for sale.
The calculator above will tell you how to set your switches for either model of Speedcal and for either model of 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.
In order to calculate your current VSS pulse per mile rate, you will probably want to use this Mustang VSS calculator: Ford VSS Calculator
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 Calculatorwill 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
You can copy the values from these calculators and paste them into the VSS calculator, then copy the value from the VSS calculator and paste it into the Current Setup for this calculator.
For those of you who want to understand how it works, Speedcal units need 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, assuming it scales far enough for your setup.
The old Ford Recalibrator simply represents the scale factor as a binary number that represents the scale factor in integer percentage form from 0 % to 255%.
The new Ford Recalibrator represents the correction factor as a 1.2 decimal number called the "Ratio Change" that is the inverse of the scale factor.
Keywords: Speedcal calculator, Recalibrator calculator