Can you recalibrate your speedometer using the road speed constant, going from 3.42 to 3.73 gears. Or have to change the speedometer gear. Never mine I believe I found the answer which is no.

For that particular change in gear ratio, you cannot change the speedo display via changes to the eprom (the bit switches work but only on MAJOR changes like using a T56 tranny).

The "Road Constant" changes the "internal speed" within the ECM itself, such as when the ECM is displaying a slightly different speed from your speedo. Your speedo stays the same, but the vehicle speed displayed by the Scan Tool will change with the Road Constant.