Premature rear tire cupping and size differential. on 2003 Ford Focus

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Rear tires cup prematurely on the inside of the tires making it necessary to buy a new set of tires every year. I have the ZTS model with P205/50R16 omni-directional tires (low profile). My nephew has the SE model that has P195/60R15 bi-directional tires that are a higher profile. Why the difference? Car has been aligned yearly. Tires rotated regularly. Mechanic says "no problem". Any help?
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I own a 2004 Focus, having same problem. Tried alignment 3 times, glad I bought the "lifetime alignment" though it didn't help. Someone suggested replacing struts. Did this. no change. I rotate regularly so I couldn't tell if it was a front tire problem or a rear tire issue. Sounds like I am not alone. Ready to sell it; tired of buying new tires every year, too. Sigh.
Car has been killed by a deer, so no more problem with it, but I replaced it with a 2001 model which does not seem to have the same problem so far. I also have a 2006 model which also does not have the same problem. I think a design flaw in the 2002 to 2004 models?
We have a 2001 wagon that has the same problem. Ready to give up on it.