Howdy,
I just recently got an 89 Burb and I'm thinking of mounting my BFG 33x12.50's on the stock 15x7 wheels. Now I understand that I have to let some air out so the tread wears evenly. I havnt stuck them on the truck yet but I want to see if they will fit without a lift on the truck. I dont want to spend the time getting them mounted to find out they wont fit. If anyones got any info let me know. Thanks  

The rear is not a problem for clearance. The front might be depending on the state of your front springs. BTW, I own a 89 burb and have since it was new and if you put 33's on it, it will be a bit of a slug with stock axle gears because it is heavy and gears are tall for those tires.  

they will fit on your rims but you should NOT decrease air pressure to "even out tread wear"! first, because i don't really think that will be a real issue, and second and most important...you run a serious risk of overheating them. without getting into a whole tire engineering lesson, think back a few years to the Ford Explorer/ Firestone tire mess! Underinflated tires will get ya hurt! Air 'em down off road, then blow 'em up to required minimum psi before ya head for home. I am telling you this as a guy who works in the tire industry.  

Alright thanks gents. Like how much does it slow the Burb down with the 350 in it?  

you should contact the tire manufacture and find out the recommended PSI not the PSI on the tire  

CedarburgMudKing:
Alright thanks gents. Like how much does it slow the Burb down with the 350 in it?

A good bit. It is not for lack of motor but lack of gears. OD will be pretty worthless and it can shorten tranny life. WIth a set of 4.56 gears it will move out smartly with 33's and still cruise nice in OD. OD is pretty tall on that tranny (.7) and even a gear that might look deep is pretty tame in 4th gear.