I would like to build an as-much-portable device for measuring the speed of a vehicle, to check speedometer accuracy and know the offset if one present. The idea is a cigarette pack sized box one can take with when test driving a car, for example. This is nothing to do with very high accuracy or racing speeds.
My first ideas range from very a very basic, but very tricky to mount, contraption with some sort of switch that revolves on a wheel, and those revolutions per second are factored with the wheel diameter, to get speed. This metric would be communicated via radio or IR to the display unit.
At the other end of the complexity scale is an accelerometer, no mounting, no wireless connection, but the circuitry would be a good deal more complex.
Is there any other way of doing this.