In my lifetime have done 3 "GNX" dashes the last 2 were using the VDO electronic programmable speedo and there are no cable or gear calibration issues. You simply measure out one mile; go to the beginning, hit the button on the speedo, drive, and at the end of the mile hit the button again, and the speedo is calibrated-- if you change tire sizes or gears you can recalibrate easily. You will need a VSS buffer from Casper's to feed a signal into the ECM or get a chip to not use a MPH signal. I prefer the VSS.