mbeach, please take a little time to review your research. You'll find the correct answers to many of the questions you brought up. I don't wish to get into any arguments or unpleasantness over readily available facts. Look into ECU's in general as a fundamental understanding of what they do and how they work will help you a great deal.

As for your wish for large tire speedometer calibration, you may want to check with St. Charles Isuzu. I'm not certain if this is available for the VX, but should as there was a "large-tire" gear for the Trooper's speedo sensor. As I understand, it was available for folk running 31X10.50 rubber. Failing that, or if you want more adjustment resolution, you can get an electronic speed calibrator that fits inline to modify the speed sensor signal from the transmission: TruSpeed Signal Recalibrator.