I just tried the Serial.print('\r') but it doesn't work in the serial monitor. As if the "\r" is ignored.
That's why I said "if your terminal application is behaving properly."
The serial monitor isn't a true terminal application.
Try using some other terminal program e.g. hyperterm on windows, minicom on linux, not sure on mac OS X; I think ZTerm is popular. I use kermit, but then I've been using kermit since I got it for my C64.
Oh, and my example may not be quite right - you may want to use
so that you make sure you overwrite the old values all the time (e.g., if the old value is 100 and the new one is 99, you could end up seeing "990"). Just use enough spaces between the "\r"s to make sure you overwrite all old values.