OK, I'm stumped. I just bought a ScanGauge, got it hooked-up and all readings seem accurate....except that the speed it registers is 5-10% lower than my speedo reading. Even the tach is spot on. Wouldn't the OBD II port be connected to the same place (computer, ecu, etc) as the VSS? What's interesting is that it is not a linear % difference, but exponential. No biggy but always looking for something to fix. It's the OCD.