Pioneer made a device that does exactly what you want. The problem is that it was discontinued, and for my way of thinking, was kind of clunky anyway:

Pioneer ND-PG1

Click this link:
http://www.pioneerelectronics.com/PU...ssories/ND-PG1

Not much info but there is a picture

Easy Installation
Magnetizes tire's steel belts
Speed detection by magnetic sensor
Highly accurate distance calibration

You might be better off using an ABS sensor/clock ring (or the signal off the existing unit?) but you're going to have fun figuring out how to trigger the activation of the lift motor...

The wiring of the components for the Crossfire are documented but not to the circuit level.