Wheel backspacing/offset for 15x8 rims

[VonFriedline]

Hey everyone,

I've been Googling about this so much that I'm giving myself a headache, and can't find any good answers. Hopefully someone here can help me out.

I've got a stock 1988 K2500 Silverado that I'm planning to put some larger tires on. I'm going to be putting BFG Mud Terrain 33x12.5r15s on Pro Comp 15x8 with 6 on 5.5 bolt pattern rims. I realize that I may have to do some fender trimming which I'm fine with.

My question is, what kind of backspace and offset do I need to go with? I can't find the offset for the stock 16 inch wheels to save my life. I don't care about fender trimming and possibly installing flares, but I don't want the tires to rub on the inside.

Pro Comp has 2 rims that I'm looking at:
Rock Crawler Series 51, 4.25 backspace, -6 offset http://www.4wheelparts.com/Wheels/Ro...l_productSpecs
Series 51, 4 backspace, -38 offset http://www.4wheelparts.com/Wheels/Se...l_productSpecs

Please help, I don't know what to do!