If tread separation caused accidents, there would be overturned semi trucks everywhere. Somewhere I read that most tire treads you see on the road are from cars, so there would be even more crashed cars.

The point I think needs to be taken from this is to keep your spare tire fairly new. This comes from experience. After changing a flat a few years ago, my spare threw the tread within an hour, far from the nearest town.

It is possible to drive miles on the bare steel belt without the tire going flat.

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1983 300SD "Guderian"
1987 MR2
2015 Camry
2015 Chevy Spark
2006 Hyundai Tucson