I know there are several queries similar, but the search didn't quite answer this for me. Am buying a 2005 model, and the rear tires have about 4mm of tread on them -- how many miles should I expect out of these before having to replace? They are Bridgestone 255 35s. I am thinking if getting less than a couple thousand miles should be negotiating a discount for something like half the replacement cost of them..any thoughts? Thanks!!