It happens sometimes that car speedometer fails and you have two choices – either try to fix it or build another one that would be different and cool. The following project is built around AVR ATmega328 microcontroller. And yes…its Arduino.
Speed signal is taken with hall sensor that is triggered by magnet placed on prop-shaft, the rest is left for microcontroller where it can do much more than display current speed. It also displays max speed and trip distance when car isn’t moving. So this can be a perfect replacement of broken speedometer or nice addition along good one.