Your spare rim is an 18". A 255/70/18" is .1" taller than a 275/55/20. These are standard size tires on a Jeep JK Sahara. They are for sale cheap in all the classified ads with or without rims. Your dealer is FOS!
Anyone upgrade to the larger size spare? What fits in the spare well??
There's a guy on the other GC forum running 18" wheels and tires from a JK Sahara which are 255/70/18" and says the spare fits inflated. I plan on getting one as a spare but haven't yet. I put 275/55/20 GY Silent Armors on my wife's Overland.