ok my take as a former tech AND also a former store manager of a firestone store in a earlier life ... can be caused by any of several above problems the most likely 2 are out of the above are...align / ie toe/ loose wheel bearings etc ... and tire balance next choice would be tire pressures biggest problem here is once a pattern is there its there the fix is new rubber along with the necessary repairs ,used to see this a lot in small front wheel drive imports AT mostly the rear tires caused by above problems in order mentioned above
couple of links ....
http://www.procarcare.com/includes/c...dtirewear.html
and ..
http://www.justtires.com/services/commonWear.html
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