Tires: 20 8.5 rim, fender wells, rim width

Question
I have a 2006 silverado quad 4x4 and I am looking to buy a 20" 8.5 rim.  I
want to know if a tire 305/50 R20 will fit this wheel or should I buy a wider
rim?

Answer
Matt,

8 1/2" is the minimum rim width for a P305/50R20.

But the bigger question is whether there is enough room in the fender wells for that size.  Unfortunately, the folks that measure that stuff think that information gives them a competitive advantage, so they don't share.

But I think it is best to work with someone who is going to supply you with the whole package and can assure you there will not be any rubbing and the load carrying capacity is adequate.