Just signed on, attempted search for an answer, unable to find. Tomorrow I may have a Durmax w/Allison. Was told by a dealer if I increase the tire size, I may cause damage to the Allison. Apparently because of the braking feature with the transmission.
Totality blew my mind, all my 4x4, first thing I do is fill the wheel wells with rubber. Appreciate some feedback ASAP. I will be towing a mid sized 5th wheel a few thousand miles each year. Thanks Ernie

http://dieselplace.com/forum/showthread.php?t=264

Thanks for the quick comeback. I did locate that thread after I posted here. Seems like most were supprised as myself. I could understand have shifting problems possibility, ABS etc. But have you or anyone noted that the Allison had substained damage from oversized tires under normal operating conditions with 1 or 2 sizes over??? I bring this up cause I was about to buy my first GM /Allison and the dealer (small shop) informed me, a customer was denied warranty repair due to oversized tires. He didnt recall the tire size but stated the repairs cost $3500 to the individual!!! I was planning on installing 265 and the intent is haul a 5th wheel a few thousand miles a year. Not sure I want to go with the Durmax now. Sure was impressed by the vechicle. Again thanks, Ernie

That dealer is WAYYYYY un educated if they blame 3500 trans damage to slightly oversize tires....

Better shop dealers Ernie

:grd:
I installed Michelin LT265/75-R16's - no problems after 10,000 miles. Towing a 10,000 lb fifth wheel.

2004 GMC 2500HD Ext. Cab Long Box ; Duramax (LB7)/Allison; GMC bed liner; Air Lift Super Duty air bags; Michelin LT265/75-R16 LTX M/S ; Luverne Running boards; Bilsteinshocks; Isspro gauges; Racor frame mount fuel pre-filter.

I put 285's on mine with 45 miles on it. Now has 55k and still fine. Granted, its not stock anymore..

Running 315/75/16 BFG AT on my rig. No problems here but using HyperTech to re-cal the speedo.

Thats funny the dealership would tell you that. I live here in nampa idaho. And the edmark superstore chevy/gmc. They put aftermarket rims and tires on these trucks 1/2 tons 3/4 and 1tons Some of the trucks they have they put 285/75/16.And lift the torsion bars some even have 295/75 /16. So i would not beleive the person who told you that. And go ahead and fill those fender wells! Wont belong before i do.
Cody