Need to reduce frequency by factor of 10

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Joined Mar 16, 2014
I am working on the speedometer in my 97 F250. It currently uses a frequency generator type thing in the transmission, which if I under stand correctly creates a pulse which corresponds to 8000 pulses per mile. I would like to use the differential speed sensor which ford switched to in 99. This sensor currently should be putting out a calculated pulse of 79320 pulses per mile. I calculated this using 120 teeth on the tone ring and 661 tire revolutions per mile. I found some information about how to calculate the frequency.
PCMs and speedometers/odometers expect an 8000 pulse per mile signal. To convert from frequency (pulses per second) to speed (miles per hour), the following equation is used:Speed in M/H = Freq in P/S * (1M/8000P) * 3600S/H = Freq * 0.45

I would like to build a circuit to be able to correct the frequency for accurate speed. I am not really sure where to begin or how possible it is to do. I do know that there are some aftermarket calibrators that do something similar for mustangs but the frequency is closer. I have repaired a good bit of electronics but never designed one. Any direction or assistance would be greatly appreciated.


Joined Oct 2, 2009
The owners of All About Circuits have elected not to allow discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

This thread is against the AAC forum rules, Chapter 6, as seen here:

6. Restricted topics. The following topics are regularly raised however are considered “off-topic” at all times and will result in your thread being closed without question:
• Any kind of over-unity devices and systems
Automotive modifications
• Devices designed to electrocute or shock another person
• LEDs to mains
• Phone jammers
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This can be found in our Terms of Service (ToS)

Here is a site that may provide you with some assistance: Electro Tech Online

Good luck.
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