[Al Skierkiewicz]

John,
Most teams use some form of sensor that counts wheel rotation or fractions of a rotation. These can take the form of an optical sensor that closes a contact each time it sees a change (from light to dark or dark to light) and a segmented field, mounted on the wheel. Two such sensors, on a wheel with eight black and eight white segments (16 transitions in 360 degrees) can give you accuracy down to 1/4 of an inch (under a cm) and direction information. Teams were given gear sensors in their kits this year that also did a nice job as well but you need to have metal gears and a place to mount them. In either case, they are not an analog output. They produce digital pulses which can be read by any digital input.