If you don't have the performance pack with the big Brembo brakes, just about any cheap tire/wheel combo with a similar diameter will work to get you home. It won't fit in the spare tire well but you can just throw it in the trunk.
I have chosen 19-inch wheels and Michelin PSS tires with the 40 aspect ratio to replace the OEM wheels/tires on my performance pack GT. In my view, the advantages are numerous:
substantially reduced cost over 20's
more protection from potholes for expensive wheels
better ride quality...
He's already a full size wider front and rear than Ford's own choices for the performance pack and about a half-inch taller. Also, the PSS tires run wider (throughout the line) than the Pirelli P-Zero's that come with the performance pack, measuring more like 275- and 295-width tires from most...
These comments represent sound general advice. Actually, I want my rim width to match my tread width as closely as possible within my chosen brand line for the same reasons stated by this poster.
Michelin's own published rim width range for 295-wide PSS tires is 10.0-11.5. My preference would be 10.5 on the rear of a staggered setup but you should be fine with 10.0.
20 X 10.5 +45 is a tried and true wheel spec on the rear of the new Mustang. You are definitely not pushing the envelope in terms of width or offset. It is your tire choice and your personal taste that will dictate the look within your fender well. Forum member Brian@Vossen went with...
I get the 10.5-wide rear wheels but why not offer 20 X 9.5 for the front of a staggered setup for those of us who wish to go wider than the stock Performance Pack specs both front and rear?
Can the forum members confirm for me that the proper selection to replace the OEM lug nuts on a Performance Pack GT coupe is the following?
Gorilla "The System" Acorn Black Chrome 14mm X 1.50 5-lug #71643NBC