How does the ScanGauge II detect actual MPH?

It reads it off the ECM which is calculated from the VSS.

Thanks for the input regarding the speedo. After changing my tires my wife was using the truck and noticed a big difference from the speed trap signs (digital readouts). Then I had the opportunity to drive a "newer" rig and the trannyshifts felt totally different. After driving a while I started noticing a drop in mpg.

This gauge is controlled by the instrument cluster circuit board based upon the cluster programming and electronic messages received by the cluster from the Powertrain Control Module (PCM) over the Chrysler Collision Detection (CCD) data bus.

The Smarty can also recalibrate it for Tire Size

Right, but if it just reads from the VSS, it must get the same signal the speedo does. In order to correct for tire size there has to be a way to tell the computer the tires are larger. The ScanGuage II allows you to do this?

it getsin information from the PCM. But like in all the rigs I used it in the ScanGauge II was dead on the money compared to the speedometer. If the ScanGauge II is off you can calibrate it too.

:sos:i just had a dealer recalibrate mine for 285s and now my truck takes off in 2nd gear instead of 1st. now my truck is pulling 2 codes,p1693&p1698 dealer told me these are chrysler codes that control the trans. if anyone has any idea how to fix this issue without spending big $ i appreciate any help you can offer.