My 2010 I picked up in September w/1,600 miles had the original M21s.  The rear was pretty well shot. After another 600 miles, I took it off and mounted a 606 a friend had left with about 15% tread left.  The MT21 rear was a slick at 2,200 miles.  690s are ruff on rear tires.  I have a new tread waiting for the 606 to terminate.  It will be interesting, a Motoz Tractionator: