I have an `01 Durango R/T and I want to get new rims and tires. I've found some 20" rims but I'm wondering if I change the size of my rims and tires, how bad does that screw up my spedometer and AWD?

 

As long as all 4 wheels have the exact same size wheel dimensions, your AWD drivetrain will be okay.

As far as your speedo calibration, it is hard to say. Upgrading to 20 inch rims and getting low profile tires mounted to them may have very near same size total diameters as a 17 inch rim with 60 series tires mounted on them. So the speedo calibration will not be off that much if any at all. Just depends on what profile tire sizes you will go with to mount on the 20 inch rims.

 

http://www.durangoclub.com/faq/DurangoTireCalc.html

Good site for tire sizes and MPH. Hope it helps.

 

You could get a Hypretech Programer III and change your speedo.