the policies at Costco have changed a little.
we can mount almost anything now, provided that they are not too wide or narrow for the wheels, they do not rub, but again, we can only test what it does on flat ground, so if we think they are too close and will rub with articulation, we will not mount them.
even if we do mount them and they rub with articulation, the tires are still yours(which I think any realistic person would be OK with),
the tires also have to be able to hold enough weight for the vehicle(we will use the placecard of the vehicle).
and we have to be able to inflate the tires to what the place card says(so if the vehicle calls for 80psi, and the tire only goes to 65, it would be a no go.)
I do know that a lot of the bigger tires will still be able to hold the weight of the vehicle plus all the stuff on it at reduced pressures, but that is one of the rules.
this one is where larger trucks(3/4 and one tons) get stuck, as most larger tires(width over 295mm) can only be inflated to 65 or even 50 psi. now, the tires will usually be able to hold the weight of the vehicle(eg, dodge one ton, oem tire are 265/70r17 LRE, calls for say 50 front and 70 rear, a 315/0r17 LRD will hold the weight of the vehicle no problem, but we can't inflate them to past 50PSI, even though I would likely run them at about 35 front and 45 rear(will hold close to the same weight as the stock tires do at oe pressures, and they will still ride very rough).
we mainly use our best judgement.
Costco does also does not warranty tires that are damaged while offroad(or caused from running the tires aired down, etc.) kinda sucks, but I think it is understandable, being as that is always a possibility while wheeling.
this is mainly to reduce the amount of money we loose if we warrenty the tires, offroading has risks and costco big wigs feel that costco should not have to take the risks if people decide to go wheeling
I will clarify with the boss what the deal is with carry in wheels, because I think they don't like us to do a lot of carry in's because that is usually where people know they will not fit without rubbing and what not.
A lot of the time we get people that want us to mount tire that are outside of the tire specs(rim too narrow), and we have to say no.
hope that helps,
Steve