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I build this speedometer for my own motorcycle. I am using it on my own Honda CD70 motorcycle (70CC) and it is working pretty well. The concept of this speedometer is very simple. A reed switch is used to measure the RPM of the wheel, and some mathematics is used to determine the circumference of the wheel. And that is then used to find the distance traveled in unit time, which is called speed.
In this project only three digital I/O pins of Arduino are used by the Display. And all other pins are available for future addition of functions, like engine temperature, fuel gauge etc.