According to Tire Rack the OEM tires are 265/70/18 rated @3525lbs
The optional 265/60/20 are rated @3195lbs. If that matters.
Price wise the 18's are $208 and the 20's are actually cheaper at $171. Both E rated of course.
The 18's are 31.7H and the 20's are 32.5H
http://www.tirerack.com/tires/tires.jsp?tireMake=Goodyear&tireModel=Wrangler+SR-A&sidewall=Blackwall&autoMake=Chevrolet&autoYear=2015&autoModel=Silverado 2500 HD 4wd&autoModClar=Crew Cab&partnum=66SR0WSRA&tab=SpecsNot sure what the OEM brand is as sometimes they change.
Interesting that the tread depth for the 20's is 16/32 and the 18's are 13/32