Whats the tallest and widest tire I can put under my 03 HD2500 with no mods?

I would say a 265-75

If you go to a 285, your gonna be rubbing most likely and that would require some trimming or t-bar cranking, which may be considered a mod.

I have BFG 265/75/16 AT KO's on by truck all stock. Fits great.

I believe that I read posts saying that the rim width on the 2500HD is 6.5 inches. Some 265/75 tires I looked at were spec'd for at least a 7 inch rim. To meet spec, I would have to upgrade the rims (assuming the stock rim actually is 6.5). You might consider this a "mod".

However there were also posts saying that they were running the 265/75 on the 6.5 rims. I'm inclined not to do this, especially after the Ford/Firestone failures where Ford did not follow the recommendations from Firestone (on air pressure).

It is ok to run 265 Revos on stock rims, however, I am insterested in getting 285's and noticed many of you run them on stock rims. Have you noticed any adverse effects (tire wear in the middle of tread) to this? Did you have to negotiate something with the tire retailer (voiding warranty...) to persuade them to put an oversized tire on the stock rim?

Per Bridgestone, the 265's minimum width is 7.0 inch and the 285's minimum width is 7.5 inch. I have the 265 Revos on factory 6.5. Also, I have seen 285s on the factory rim.

However, with the narrow rim of the 6.5, the tire's foot print somewhat pinch as you will lose traction benefit of the outside shoulder tread. So, the wear pattern may result in more wear in the center tread area versus the outside tread. Also, as mention before, a little trimming of the front air dam might be necessary to allow for clearance.

Please correct me if I'm wrong......But it is my understanding that the 265's that come stock on the 3500's are mounted on 6.5" rims.

I am running 285's on stock wheels, this is my 3rd 2500HD that I have done this with. Along with countless friends that run the 285's on the stock wheels. Is it really what should be, no, but you shouldnt put programmers, off road, etc... I have never had a problem and have not heard of any problem. You can not run factory tire pressure though. The 245 can handle 85 psi, were like my 285's max at 65 psi.