Having recently sold my 2013 Avalon LTD I had a spare 18" wheel & wondered would it fit my '13 GS350. I mounted it on the rear & it did fit. Granted the spokes are different but keeping it in the trunk just in case will work.

Doesn't your car already come with a spare under the trunk board/floor?

Should also consider the size of the Avalon tire/wheel as it might not be appropriate for the car, even as a spare.

The car came with a donut, Having a full size spare doesn't limit you to 50mph.

The Avalon wheel is also 18" as stated so the only difference is in the spokes not size. This would give me time to get another tire on the OEM wheel. Just making use of what I already have.

Check the front as well... caliper clearance. Not the way I would go, but to each their own.