How to re-calibrate a speedometer

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Many cars have a speedometer that overestimates the actual speed. This is usually made on purpose by the constructor under the pressure of the government, in order to make the user drive slower than the actual speed limit. Car dealers usually refuse to re-calibrate your speedometer, and should they accept, you would need to have a big wallet to pay for the bill. It is actually very easy for them to reprogram correctly the speedometer, but that procedure is kept secret, and it is hard for DIY'er to guess what the procedure is without all the required information on the microchips and and their firmware.

In order to solve this problem, I propose in this project to build a simple eletronic circuit that intercepts the signal that is sent to the speedometer, modifies it, then forwards it to the speedometer in order to make it believe that your speed is smaller than what it really is. Then, the speedometer, by exagerating the speed, actually outputs the correct speed.

In the video below, I explain how this can be done:

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