Originally Posted by Gwagensteve
The speedometer doesnít know what gear you are in. Just the same way the speedo doesnít change its reading when you change gears in the gearbox or change ranges in the transfer case, it doesnít know that there are reduction gears in the transfer case. It took me a while to work this out myself. Yes, it corrects revs at road speed, but NOT the speedometer, because itís downstream of the reduction.
So where is the speed sensor and what does it use to pick up the speed? I would think it measures the rotation of the drive system either in the tranny or transfer case or tire rotation in the axle. I doubt it's in the axle as I don't see anything there. My thought was if it's using rotation somewhere within the driveline, a larger tire would cause the speedo to read low but that is offset by a lower gearing. This is an interesting topic as I am learning something new.