I wouldn't go modifying the car. It certainly wasn't designed to wear tires unevenly.

The first thing to check is obviously tire pressure. I expect you had that within spec. Next is toe. Too much toe in causes uneven wear on the outside of the tire. Then camber. I don't know if the PT Cruiser has adjustable caster, but that would be the last of the three alignment specs to check.

If it drives pretty stable both at speed and in turns, it's gotta be too much toe in.

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1984 300TD