Kevin is correct.  Putting a magnet on the drill shaft and using the magnetic speed sensor that came with the Calc should work.  You would need to mess with the Cal number to get the correct speed.  Once that is done, the acres will work out correctly if the correct width is used.

If the Cal worked with an old real radar gun it should work the same way with the GPS radar simulator device. This type of device was created to replace a radar gun so they use the same wiring.