More Accurate Speedometer

I’ve been sitting on this idea for at least a year, and I’m not really sure why I haven’t blogged about it yet. It turns out that car speedometers are not very accurate. In fact, they can be on the order of 10% wrong, with the error more prominent at higher speeds. Typically they work by counting the revolutions per unit time of the axle, and then some quick math involving π. The odometer is usually measured in a similar fashion. I think that this is a terrible way to measure speed and distance. A much better way would be to have a photo-detector scanning the surface of the wheel. The system would work on the same principles that enable optical mice to detect their motion. It would be more accurate because there’d be less variation in tire width, and error would not compound at higher speeds. The only real drawback is that mud or dirt might obscure the sensor (but then you can fall back on the existing methods).