# Need to reduce frequency by factor of 10

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#### makonkey

Joined Mar 16, 2014
1
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
from http://www.accutach.com/Documents/VSSCharacterization.pdf

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.
Thanks

#### MrChips

Joined Oct 2, 2009
23,221
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.

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