Well, Jeep went to Hall Effect sensors that read off of the shaft revolutions, assuming a particular tire diameter and diff ratio, rather than the mechanical gears of the pre-electronic speedo's. (I loved being able to simply swap in a new speedo gear for a few bucks and be done with the recalibration...)

The Hall Effect sensors should be a three wire sensor reading off the shaft, and sending info to the speedometer and odometer...the tone rings sensors (two wire), should be sending info regarding wheel speed to the ABS and traction control, etc.



That means BOTH gears AND Tires, impact the speedometer/odometer.