Well I finally got the tires mounted through Costco. I had tried lying to them that the wheels I was bringing them were off a 2500 silverado so that they'd not give me a hard time about the size difference. My hope was that they'd be so unaware of the 6-lug vs 8-lug difference that they would never know. I think the service manager at my local Costco Tire Center doesn't like me, so he went out of his way to look at the wheels and determine they weren't the right ones. They said they'd mount the tires and explicitly write on my "file" that they have no warranty through Costco. So good for them on protecting their giant corporation I guess, and at the end of the day it's whatever. I tried what I could, I can't be mad.
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I think the 275/70/18's look great! Fills out the wheel wells much better than the factory tires, and no rubbing even at full lock.