I have 20's. My 2011 Yukon xl has nice 20" rims with 275/55R20's. My spare tire is a full-size tire, but it is on a generic 17" rim. The overall wheel size is virtually the same.
I read the Owner's Manual, and then I read some stuff on the internet and I got nervous. I tow a lot (boat and TT) and we just came back from the snowy mountains. I usually only need 2WD but sometimes I use the 4WD (on road, not off-road since they're 20" rims). The reason I got nervous is that I heard that if I ever get a flat and my spare has a different size rim, I can't use the ABS and traction control. I understand this would definitely be the case if the spare's overall tire diameter were different, but I don't fully understand why the rim size has to be the same. Maybe it's not true and I'm overthinking it.
ANYWAY, for fun, I bought a nice matching 20" rim online and had a 275/55R20 put on at Costco, so I don't have to worry about it. And YES, it fits perfectly well in the spare tire area. (I went to the local dealer and the two parts ****'s all but kicked me out when I tried to buy an OEM rim from them. They insisted it was a dumb idea and insisted it wouldn't fit in the spare tire area (which already housed a full-size spare). I stood there showing them the page in the manual that addresses it, and politely asked if I could just order a rim. The guy said "You have a nice day sir!" I'm not here to whine or rant. My interaction with them wasn't heated, but I thought it was comical how the guy was reluctant to sell me a ($500) rim because HE thought it was a dumb idea. I'm not mad; I just hope they end up reading this..... "It fits perfectly fine!!!"
If you guys get a matching spare rim/tire, I got a really nice used/refinished one online much cheaper than if the dealership had allowed me to buy one. It looks new and while I know there's a chance it could have a hidden defect, I have more confidence it it than the original (6-yr old) 17" rusted out rim that was under there.
BTW:
We went to the mountains in a snow storm last month and I bought tire chains for this 2011 Yukon. They fit great, but I didn't need them; that Yukon crawled through steep, snowy inclines effortlessly. The Owner's Manual says something like "Don't use chains, but if you do, put them on the back, not the front. But if you put them on the front and back then follow the chain manufacturer's recommendations closely." The chains I got are low-profile, self-tightening, self-adjusting, cable/chains. Easy to put on. But I didn't need them.
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