I was just wondering if a pair of BOSS 301 20x8 rims will fit on Goodyear Eagle GT II tires, 275/45 R20. Thanks

Are the 20"s for your X?
If they are, a search wont help, not on this board anyway. Not alot of people drop twentys on Xterras. You could ask tirerack.com.
Hope you have an asbestos suit handy...

Well, to answer the EXACT question, the tires will fit just fine on those rims.

As for the implied question: I'm assuming you have a Frontier from your screenname. Those tires will be just a hair short of 30" tall on those rims. Fitting those on a 4 cylinder 2wd Frontier will be next to impossible. As for the 4x4 and V6 Frontys, if you have a 2001 and up, it should fit fine as long as the rim has the proper backspacing. If you have a 98-2000, you may rub the mudflaps depending on how much those rims stick out. And that would be a matter of trimming. Check the tire shop and see if they will let you test fit.