The factory spare is a P235/85R17 full size on a 17" steel wheel, temporary use
..same diameter (32.7") as a 275/60R20, just not as wide
235 vs 275 =40mm ...about 1.5" less width.
it weighs about 75lbs almost the same as stock 20's w/tire (80lbs) ...for those like me who like to drop it for racing weight reduction.
You can use an identical stock 20"wheel/tire for a spare if you pick one up on ebay or the local salvage pick-your-part.
...but the center bore hole may not allow the tire T holder to pass through (for those with Chrome Clad wheels) without some trimming.