Hello,
Had a tire go down after hitting a curb and the tire was cut. Not repairable. Good thing I have road hazard coverage. However, major issue was this occurred on a Saturday nigh in San Marcos, TX and could not find a place that sold a tire i needed. 265/40/r21. Had the donut on and needed to get back home which was over 250 miles drive. Not good on a donut. Luckily I was able to find a used 265/45r21 and made the trip home with no issues.

My question, is can I use a different rim size for a spare? Say a 265/45r20. Based on tiresize tire is an almost equal match. +0.3% diameter.

What does the forum member think?

2020 Edge ST does not have the brake upgrade

 

Honestly I would have no issue going 200 miles on a donut spare assuming the vehicle is not loaded down.
I believe you are asking if you can get a full sized spare that the is a slightly different size then the four regular wheels.
Its not great to ride on different sized tires if they share a axle, if you are front wheel drive only and put the slightly smaller spare on the rear I do not see that being a issue.
Your existing stock spare tire is probably not a exact match in diameter to the other four tires.