2003 Kenworth T600 with odometer stopped

It's the speedometer. You will need a new one. I have a 03 that did the same thing.

 

It's Electronic, do a Google search online as there are places that can most likely rebuild it.

 

Same thing happened to my 1998 a few years back. Bought a new one. Don't forget to note the position of your old dip switches so you can set the new ones.

 

Tallmon here is a website that sells speedometers which are cheaper then the oem ones. Only difference is no KW symbol on the face of it. I think you can also let them know your miles that was on your old spedo and they will set it to whatever you want. They might even repair your old one if you like.
http://ddinstruments.com/5-inch-kenworth-speedometer/

 

My local KW dealer said no when I asked that question. Maybe SAR's link is a better option.

 

I've never seen a dealer set mileage on a new speedo. Google "speedometer shop", it's surprising how many places do it. Companies that specialize in fixing old gauges for restorations can help you out. We used to use a guy in Michigan at my last job, http://www.bobsspeedometer.com