# Electronic pulse frequency modification (vehicle speed sensor)

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#### Apple_Eater

##### New Member
Hello,
Firstly, allow me to apologize in advance if my post is not accurate or in the wrong area. I have a very limited knowledge of electronics in general but am interested in learning. With the disclaimer out of the way, here is my issue:

I have recently switched the tire sizes on my 2001 LandRover Discovery. This has caused a discrepency in the speedometer on the vehicle vs. the actual speed being traveled. I am looking for an electronic, rather than mechanical, fix to this issue.

According to the repair documentation of my vehicle, speed is picked up by the speed sensors above the wheels, which goes straight to the ECU. After reaching the ECU, some magic is done, then the speed is transmitted to the instrument cluster in the following manner:

The Self Levelling and Anti-Lock Brake System (SLABS) ECU provides the signal input for the road speed. The signal is at 8000 pulses per mile (1.6 kilometres).

My assumption is that the ECU uses the speed sensors above the wheels to calculate the actual distance being traveled (for odometer and speedometer operation) and will pulse whenever it determines the vehicle has moved 1/8000th of a mile. As my speedometer accuracy discrepancy is around 10% (I will obviously do more testing to be more precise when the time is right) it will in actuality be pulsing for every 1/7200th of a mile.

Here is my idea/question: Is there some sort of electrical device or easily-obtainable (read: cheap) chip that can take an input frequency in the 8-800khz range (to account for all speeds from 1-100 MPH) and reduce the frequency by 10%, then output the result on the same wire?

If there is such a device, will it be able to run without any external power source (using the voltage in the wire it is modifying) or will I need to power it somehow?

Also, if my idea is impossible/impractical, is there some other electronic method I could use to cause the vehicle to report the correct speed?

For the curious, I am interested in doing this electronically so that it may be modified in the future if I again change tire sizes for some unforseen reason. I do realize that this fix will only fix the speedometer, not the odometer, but I am not terribly concerned with that.

I am looking forward to your responses and to learning a bit more about electronics during this project, and any help/advice is greatly appreciated!

##### Banned
You should contact a shop that replaces tires, there is either going to be a way to simply and easily reprogram the ECU or even possibly dip switches to allow for different tire sizes.
This is not something you should start off as an early project as there is already a system in place to do it normally. You're not the first person to get larger or smaller tires on a car and need their ECU and odometer readjusted. Don't make work for yourself that a proper solution already exists for.

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