What treadwear rating printed on the tire is determined by the manufacturer, and there is no real standard test, so comparing different tires can be misleading. Having said that they are a general indication of how long a tire should last. My guess is that every 100 units gets you about a year of life under normal useage. The RE92's last about a year and a half and the Pzero Neros M+S's should last about four years. If you drive like a granny they may last longer, if you autoX they may only last half that long.