My 09 has the stock 265/70/17's. I'm wanting to make the change to 20's. If I switch over to the stock 275/60/20's, I assume I'll need to have the speedometer adjusted?

 

Barely. Here's a calculator where you can see the diff........... https://tiresize.com/calculator/

 

And that same site has a calculator showing how far off your speedometer will likely be ..... https://tiresize.com/speedometer-calibration/

Please also how much weight your new wheels and tires will be adding and consider possibly upgrading your brakes as well. Suspension components may also wear out more quickly if they weren't designed for the extra weight of 20 inch wheels and tires. If 20 inch wheels were an option in 2009 though maybe component wear will not be much of an issue. Also, I don't really know of a way to determine if braking will be impacted other than to get the lightest wheels that address your needs and to keep on eye on brake temperature and signs of brake fade.

-Rod

 

20 inch wheels were an option on 2009. Standard on the Big Horn, Sport, and Laramie. So suspension and brakes should be fine.