More specifically, is uses the signal from the ABS computer that is processed by the gauge cluster and sent to the ECU from there. With more context, we can help you with whatever you are getting at...It uses the speed from the ABS computer. I forget which input it is. I went through figuring out which speed input is used when I installed a factory NAV unit and needed the speed input to make the NAV accurate.
More specifically, is uses the signal from the ABS computer that is processed by the gauge cluster and sent to the ECU from there. With more context, we can help you with whatever you are getting at...
-Charlie