One could argue that when you corner the differential is running at different speeds normally, but you really would not want to force that constantly by driving very far with tires of different diameter. And that is the key here, overall tire diameter, not width or rim size. FWIW you should be able to find a Mustang spare wheel with a tire that is close to you road tire diameter if you want to carry a spare.
For most of us it's a tow truck call (AAA membership is mandatory with a Pantera!) if its more than a tire plug and can of sealant can't fix.
Julian
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