Does anyone know if changing out the crown wheel and pinion in a TT Z32 diff with these (to get the 4.1 ratio) has any effect on the ABS system.?
No it doesn't. The speed sensors measure wheel speed. What ever you do with the diff wont make any difference.
What he said You will however need to correct the speedometer reading by either re calibrating the speedo head or swapping in an NA speedo drive.