The penny thing works cause it is 2mm from edge to the top of lincoln's head. the tread bars are put in so you can see when you are at 2mm of tread depth. The DOT now advises people to install new tires at 4mm of tread depth. The uneven tire wear is a bad sign of what's going on with the undercarriage. I would start checking all the bearings, caster, camber, ball joints, tierod ends etc.etc. Something is worn somewhere.