2000 mustang gt abs light

Yes... because something is not working right. :D


Pull the wheels, clean off the sensor pickups (they probably got something on them while you were swapping wheels).

If that doesn't work then pull codes and let us know.
 
You need to tune the computer with the new tire size and ratio.

If you are unable to communicate with the ABS computer at all then the control module is fried and you'll have to replace it.

Lucky for you I made a video describing how to do such a thing. Search my user name on youtube and you'll find it.