Oh, right. The car player's serial port could, in theory, go up to 23400 bps...
The problem is that it's a pain to get that working because most PC's don't support that speed on their serial ports, and Windows simply won't go that fast even if the serial port itself can handle it. All of these things can be worked around, but the work arounds are generally a pain in the rear.
And after all of that, why would you WANT to anyway? Anything you'd be doing that required any speed can be done much faster and with a lot less troube simply by using USB or ethernet.