From another posting:
One option that would be a match to the stock tire as far as tire diameter is a 285-65-18. Tire rack shows a few available in this size.
What would be nice about that size, is that there would be no need for reprogramming the truck, as both that size and stock size are 32.6" in diameter.
Another option would be to go to a bigger, more popular tire size. We had a customer who bought a 2011 HD with the 18's ask about bigger tires. I told him about a 275/70/18 but that would be bigger than stock (32.6" versus 33.2") and give a 1.7% difference in speed. The plus side to this size, is that availability of tires is much more vast. Tire Rack lists 24 E load tires.
Interestingly, Ford uses this size on Super Duty if I recall, and makes me wonder why GM didnt.
So...to simplify...If you would like to go slightly bigger and have more variety, go for 275/70/18. If you see what you like in a matching diameter tire, then try 285/65/18.
2011 Chevy Silverado 3500HD LTZ CCLB LML SRW 4X4 Blue Granite Metallic.
Towing a 2012 Cruiser Patriot CF335SS fifth wheel.
2005 Chevy Silverado 3500 LT CCLB LLY SRW 4X4 Silver Birch Metallic
2004 GMC Sierra 2500HD SE ECLB LB7 4X4 Sand Beige Metallic