I asked this in the offroad section but didn't get any response so maybe it would be better to ask in a GM specific section. Help out if you can:

I would like to run a 285/75/16 on my Sierra HD that has 6.5 inch rims but want some feedback from folks that's done it. I don't think clearance will be a problem, will it? Am I going to have uneven wear problems? What PSI should I run with the bigger tire on the smaller rim?

 

You can do it, we have many times. You may need to adjust your torsion bars up a little, and maybe even trim/remove front mud flaps(if you have them). Remember to get your alignment reset if you adjust the torsion bars.

As for tire pressure, I'd recommend between 55-60 psi front and rear. Running them underinflated(50 psi or less) is worse for the bead area of the tire.

 

^^Thanks for the advice. My stock tires (245/75/16) look tiny in my wheel wells now so I doubt I will need to trim anything but I guess I'll find out soon enough. Adjusting the torsion bars is to keep the tires from rubbing or to adjust how they ride? I'm getting a lifetime alignment through Firestone so I can get the alignment done anytime for "free". These are the tires I'm looking at, load range E so I don't have to worry about carrying a load: http://www.tirerack.com/tires/tires....1=yes&place=19

 

honestly i think 285's look like hell on stock hd wheels...step up to a wider 17" wheel and it'll look sweet

 

Adjusting the torsion bars is to keep the tires from rubbing or to adjust how they ride?


Torsion bar adjustment to raise the front end about one inch. It will gain you a little extra clearance for the bigger tire, AND it will increase the spring rate a little bit, which helps with the heavier tire. We've found that the NNBS trucks wheel-wells are not as wide (front-to-back) as the NBS trucks. I'm not sure which model your '07 is.

 

You mean because of the sidewall bulge or what?

 

NNBS is new new body style, right? Mine is a classic so I guess it's the NBS and has a little larger well. How much should the torsion bar be adjusted? Should I measure the clearance, then adjust, then measure again until it's one inch or enough clearance? I'm a newb to this stuff...

 

Park on flat, level ground. Measure from the ground to the bottom of the frame rail on both sides. Raise the front end up no more than one inch by tightening the torsion bar adjuster bolts. Once you are satisfied with your results, drive to the tire shop, put on your new tires and check clearance, have them adjust the alignment back into specs, and enjoy??

 

Since your truck is a GMC 1500HD it is gonna be really tight up front with 285's. The painted plastic part of the bumper is probably going to be an issue. You might have to trim the bumper some, like this guy did:
http://www.dieselplace.com/forum/sho....php?p=2928528

There is another trick you can do to keep the tire from ripping the "bumper" off. I will try to get some pics of mine and post them up because it's kind of hard to explain.

Also, there is another tire size that some guys run on the 6.5" wheels, a 255/85/16. It is basically the same width as your 245's, but nearly 4" taller and fills up the wheelwells. Only thing is most in that size are A/T or M/T tread. Here's a couple examples:
http://www.dieselplace.com/forum/sho...d.php?t=219144
http://rides.webshots.com/album/167632134EiOnHa

 

Thanks for the info guys. My buddy has 315/75/17 BFGs which is supposed to be a little more than a 35 inch tire and we put one of those on the front of mine and it fit into the wheel well without adjusting the torsion bars so there might be hope that a 32.8 inch tire will fit ok. I'd like to stay away from trimming the painted plastic if possible.

Neil, I looked at the 255/85s but would like something a little wider plus that 255 tire is even taller than the 285/75. Would it rub less than the 285 because it is skinnier though?

I mainly want a wider, taller tire for more stability on the road while lowering the RPMs a little, and of course it would be nice if they filled up the wheel wells more. For the things I am looking for and on the stock 6.5 rim, is a 285 a good choice or would I be better off sticking with a smaller tire?