Question about a specific tire size

[James066]

I am dabbling with the idea of swapping out my stock 20" wheels with a set of 18" Fuel Rebel's wrapped in Wildpeaks. Having searched through numerous threads on here, it seems that guys have been able to fit everything from 33-35" tires on their trucks. My truck is stock and I intend on keeping it that way but I am starting to get tired of the look of the stock wheels with the Michelin LTX Defenders. My question is, has anyone successfully put on LT275/70R18 tires without any rubbing on a stock suspension set up? I like that they are just over 33" but if they are going to rub I figured my safe choice would be LT285/65R18. Also with the Fuel rebel wheels I've had one tire place tell me that I need a 20mm offset and Tirerack has told me that that product number isn't showing up as compatible with my truck. I'm still a little confused on how the offset works but I want the tire to be flush or slightly out from the wheel wells. Lastly can anyone attest to how much of a change in fuel economy I should expect?