Looking to do a wheel/tire combination on a stock 05 2500hd. I've got the 16 inch aluminum with 245 combo. I'd like to keep the tire roughly the same diameter but go a bit wider...I don't want the tire to rub. Also, can I put an 18" wheel in its place? Maybe the wheel be a bit wider too?

You can go up to 265/75-16 with no mods at all, and it will take out the error that all manufacturers put into their speedometers.

Be advised that the stock aluminum wheel is 6.5 inches wide, and some tire locations, such as Costco, will not mount 265 tires on these rims because the official industry guidelines call for 7" to 8" wide rims for that size.

I have no experience with going larger diameter, so somebody else I am sure will advise you on that.

Yes you can.
If you don't want to make any modifications at all, then I'd recommend this:
If keeping the stock 16 in. wheels, go with a 265/75/16 tire size. These are the next size up from the stock 245's. It will give you roughly 1 in more in height and width.

If you want to run 18 in. wheels, I'd recommend a 285/60/18 tire. It will give you the desired width that your looking for, and again, you won't have any rubbing issues.....
Hope this helps.

so if I go with a 18x9 wheel, what offset should I look for? thanks

I don't know about 18" wheels, but I just put some 17x7.5" rims from the new Chevy HD trucks on my '05. Like you, I wanted to go a bit wider, but like k1xv said, some places won't put anything wider on the PYOs. I put 265/70R17 on my new rims. They're a little taller than stock, fill the wheel well nicely, don't rub, and don't need any lifting or cranking.

Just a different option.

Joseph

I have 255/85R16's on my stock rims. 33" diameter. No rubbing issues at all. I did crank the front bars about 1.5" too.

Put 265-75-16's on it. BFG A/T's or Pro Comp A/T's are my reccomendation. Good on the hiway, good in mud/snow (not great), and wear well. My ProComps have 30k mi on them now and still over 1/2 tread left.