Good evening, what this is is that the tire size needs to be entered into the trans control module. This way it can calculate the speedometer correctly. Here is what the manufacture says about it:
NOTE: This procedure must be performed if the Transmission Control Module (TCM) has been replaced with a NEW or replacement unit. Failure to perform this procedure will result in an inoperative or improperly calibrated speedometer.
The vehicle speed readings for the speedometer are taken from the output speed sensor. The TCM must be calibrated to the different combinations of equipment (final drive and tires) available. Pinion Factor allows the technician to set the Transmission Control Module initial setting so that the speedometer readings will be correct. To properly read and/or reset the Pinion Factor, it is necessary to use a DRBIII® scan tool.
Inorder to perform this, you will have to go to the dealer and have then perform this using the factory scanner. Let me know if I can help you further or if you need more help.