I have an '09 Chevy Duramax 2500 4x4. I've added the readylift keys and shock entenders, and it sits level. I currently have stock 16" wheels with 245s. I was looking at buying 18x9 wheels, but I'm unsure of what offset and backspacing I need? I found a set of 18x9 wheels for sale, but they are 4 5/8" backspacing with either -6mm or -12mm offset...not sure at the moment of what the exact offset is.

Will I be able to run 295s or 285s on these wheels? Any help is appreciated!

From what Ive seen and read on the new body LMM with a level kit or cranked a bit you want to run a positive offset. Maybe somewhere between a +10 to a +20 or so. I'm looking at one of the KMC XD wheels for my truck and its a 20x9 with + 18 offset so it should be ok as long as I stick with something around a 285 tire.

Anything bigger than 285's and you might run into more rubbing issues since the LMM fender is so tight. Some guys have run bigger tires and you can too, but all depends on how much you want to cut and trim...