What causes a "rear" tire to wear on the outer edge of the tire?

If it hasn't been done in a while, I would get the alignment checked/set.
Sounds like you may have too much positive camber at the rear, i.e. the top of the tire leaning out ( like this: / ).
Have them check for other suspension-related problems/wear at the same time.

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2007 C 230 Sport.

From the rear of the car, too much positive camber makes the rear wheels look like \ /

I assume the driving style it "normal." If so, the wear pattern described is almost surely a camber issue. This can be the result of bushings collapsing, or if the suspension is properly designed (and on a Mercedes it probably is) so that the camber becomes more negative as the suspension compresses, from sagging springs.

Aggresive cornering tends to preferentially wear the outside of the tire, unless the static camber is pretty negative. Stock alignment specs are meant for the "normal" driver, and if a car is driven hard through lots of corners, a different (more negative) static camber can be desireable.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

Excessive toe-in definitely increases tire wear on the outside edge of the tires. This is a very common problem on the front tires of the W124, but not as common in the rear. Usually, the rear starts to sag with age, so the camber becomes increasingly negative, which increase wear on the inside edge of the tire. The rear toe-in is adjustable on the W124, however, so it certainly could be the rear toe-in. The rear toe is adjustable with OEM eccentric bushings that many of us wish were also installed on the rear camber arms, because it would be great to be able to adjust rear camber with OEM parts.

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I just couldn't give up on my 1995 E320.

I think it might be like always going back to that same bad relationship with an ex girlfriend.
You feel you love them too much, or you are just too stupid to know any better.



Flickr slideshow of my 1995 E320
http://www.flickr.com/photos/24145497@N06/sets/72157616572140057/