I'm with randy on this one. The size difference is almost negligible, especially if the new one has little wear. If the 205's are more worn than the "new" tire then the difference will be even smaller still. I agree that in theory the smaller tire will cause undue wear and tear on the differential, but it'll never amount to much especially since these rears seem to be designed to outlive the car anyway.

I'd be more concerned about the unknowns associated with a used tire. But then if it looks good, isn't really old and it isn't the worst of the worst Chinese no-name brands then it's most likely just fine.

I'd rest easy, but have you son call when he gets home.

-Will
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