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Note that many autos have a Vehicle Speed Sensor already built in others have a Crankshaft Speed Sensor built in. The detailed shop manuals often describe the sensors as well as what one would expect to see in terms of output from the sensors.
Note also that the Antilock Braking Systems have some means of measuring individual wheel rotations. You might "steal" a signal from there - realizing it's only one of 4 wheels.
One thing we use on drag cars for the data logger is a rare-earth magnet attached to the tailshaft, set up a Hall effect sensor to read the magnet, and work out the road speed from the gear ratios (differential) and the tyre diameter.
2 things to bear in mind - The shaft will move up and down if the car has suspension fitted, and there may be tyre spin.
Second option is to attach the magnet to the front wheel - remembering that this may become airborne at times! :twisted:
You could also try taking input from an accelerometer, such as this one from Crossbow: http://www.jameco.com/cgi-bin/ncommerce3/ProductDisplay?prmenbr=91&prrfnbr=95834&cgrfnbr=501&ctgys=. I used a three axis once to build a system to gather very detailed information on my decents when I would go downhill biking, and was able to find out my forward velocity at any given point on my trip with just a little math. For a car through, some very simple math with a 1 axis would allow you to find speed.