In an ideal world, the stock 2500HD rims are not wide enough for 265 tires.
You CAN get some bulging and squirming with the right tire on the narrow rims, but not always. It depends on the tire, and AFAIK there is no magic formula to figure out if you will or you won't.
Lots of people run 265's and claim "no problems." You need to determine for yourself if the claims are credible and if you are willing to take the risk.
Ideally, you would get wider rims along with the 265 tires, but sometimes you gotta do what you gotta do to get the job done.
2002 Chevy 3500 DRW 8.1L/Allison
2000 Palomino B1500
...and the reason why I need a DRW to haul a Palomino:
2004 United 7x14 tandem axle enclosed toy trailer
2011 PJ 8x20 7-ton deckover equipment trailer
Does "I don't want to change rim size." mean you don't to go to a larger diameter rim or you don't want to change rims, period?
Upgrading to 265's was one of the first things I did but I also upgraded to a 3420 lb rated 16x8 rim at the same time. Stock rims are 16x6.5 I believe, and the minimum approved width rim for a 265/75R16 tire is 7". If you're upgrading for the load rating but not following all the tire manufacturer's spec's, than you can't really trust the published load rating. It's "probably" still higher than the stock 245's, but you can't know. I advise doing the upgrade but also upgrade wheels too. You can usually find stock 16x7 wheels from a 3500 SRW on ebay. BTW, the load rating on your stock rims is also a consideration. Unlike after market rims it's not stamped on there and to the best of my knowledge the most GM will say is 'it's comparable to the stock tire.'
Edit- PS: I didn't change anything in the trucks computer. Stock, what was 60 MPH on the GPS was roughly 61 on the speedometer, now it's closer to 59. Not enough for me to worry about.
I went one step further on my 08 2500. I put 285/75 Nitto Terra Grapplers along with Gear A;py 7" rims. My carrying capacity per tire is now 3750#. It made a 4 mph difference on my speedo. I am now going 64 with speedo registering 60.
It made a world of difference in the ride with camper on. A bit rougher with it off. Total cost was near $1500.00 for the whole setup.
I made the switch to 265's a long time ago. After visiting many truck forums I found that the guys who used the stock 2500 rims didn't have issues. I now have 110,000 on 265 BFG AT tires and haven't anything but good things to say about them. I don't get excessive squirm, and I do air them down a lot when unloaded to get even wear. I got 75,000 out of the first set as I rotated the spare also, so I expect to get about 60,000 out of this second set that's running on the truck now.
I have a 2005 gm 3500 srw - with the 16 X 7 steel wheels - I had a 2003 2500hd prior to this truck - it had the alloy 16 X 6.5 wheels.
I wouldn't mount the 265 width tire on the 6.5 inch rim. But I am a picky engineer... Even if the tire is rated higher - the rim isn't - so why bother? Okay - go ahead and flame me now!
You can mount a 255 width tire - the toyo m-55 is one that I know of... But it is a snow tire and doesn't provide the creature comforts of the michelins, etc.
Have you ever found an actual published weight rating for either the 6.5 PYO wheel or the 7.0 steel wheel ? I have searched extensively and have not found this info except to assume it by the tire size that is put on them from the OEM . I understand the recommendations of the Tire and Rim Association as far as size . I have not read or heard of a single PYO rim failure regardless of tire size even up to 315/75/16.
I have also found that with my 2 GM trucks ( 02 and 03 ) that going up one size in tire has put the speedo dead on . Many others have reported the same . This means that you are running out of warranty 3.3% earlier than you should and the mpg looks 3.3% better than actual with stock tire size , shame on GM .
On our 2004.5 Chevy crew cab LB 4X4. My original Bridgestone 245/75/16 "E" tires with 3042 rating at approx 40,000 miles as the tread was gone and also had steel showing on one tire and my weight was over tire capacity. Replaced with Michelin 265/75/16 "E" tires with 3415 rating on the original steel rims and they now have just passed 80,000 miles on them and still have approx 3/16" tread depth left. Will be replacing the Michelins with the same Michelins this summer as they will have ~90,000 miles on them. They do NOT squirm and have worn evenly and the truck handles the loads great.
Being a retired 40 year truck/van/SUV engineer, I won't have anything but steel rims for carrying heavy loads as I have seen over and over what happens over time and miles with aluminum rims when heavily loaded. Aluminum may be stylish but does not flex like steel when hitting chuckholes or large bumps etc and it compromises over time when stressed.
Our trucks are only used for carrying a TC or our Carriage Carri-Lite 5th wheel as we have other vehicles to use for daily etc driving. Our 100 gallon aux diesel tank is almost always in the bed when pulling the 5th wheel. I have a 2 ton hoist and castered load dolly's in my toy barn which makes quick short work of the changeovers from TC to 5th wheel. Our TC's are the past Lance 9.5' or the present 11'4" biggie. Both wil usually have our 2150 lb boat or the 12' - 3,000 lb "V" nosed enclosed trailer behind and the combos work excellent.
Any larger tire than the 265, a wider rim would be needed.
A superb CC LB 4X4, GM HD Diesel, airbags, Rancho's, lots more
Lance Legend TC 11' 4", loaded including 3400 PP generator and my deluxe 2' X 7' rear porch
29 ft Carriage Carri-lite 5'er - a specially built gem
A like new '07 Sunline Solaris 26' TT