Not sure about the US spec cars, but the full sized spare is an 18" mounted on a steel rim, not an OEM 20" wheel that the car comes with.
The 18" spare has the same rolling diameter as the 20", so it is suitable as a spare.
This is the same for the SRT, but the spare only fits the rear due to the brakes, so if you have a front flat, you must first swap out a rear with a spare then use the rear for the front.
Previous Ride = 2011 JGC CRD Overland
Current Ride = 2014 Silver JGC SRT
with Pano, HK and Spider monkeys.
Mods = CLSTech Ribbon Mod
Best 1/4 13.4@102Mph