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Make an Android powered speedometer for your car

One of the things I noticed right away when we got our new van was that the speedometer was slightly off. The car is used and probably had bigger tires put on at some point, so whatever, it didn't bother me. It felt like a little problem I could live with.

But the more I tried to nail down exactly how much the speedometer was incorrect, the more I realized just how wacky speedometer problems can be. This particular problem would get worse the faster the car would go. So if you wanted to go 20mph, for example, you had to point the needle at 25. But if you wanted to go 65mph, you had to point the needle at almost 80. I tested the speed using speedometer apps on several phones, and by driving past those "your speed is" signs at various speeds.

This obviously made it really hard to ever know exactly how fast you were going. Real speedometer calibration is very expensive, standalone GPS speedometers are expensive, and simply learning to live with it was not an option for me.

The solution? A homemade Android powered speedometer for $35.