lets start with whats NOT coverd. Mechanical wear (alignment being out) vandalism or abuse (jumping your truck and poping your tires). with that being said any unrepairable road hazzard( blown tire, nail in shoulder or sidewall, run flat) or defect in workmanship is coverd. Now the 100% replacemnt certificate and the standard pro-rated warrenty cover the same thing. the difference is when the tire is, let say warn half way. the pro ration would give you back 50% of what the tire cost. So you would have to pay for the other 50%. the free replacement is just that. no matter how old or how many miles are on it as long as there is legal tread the tire is just given to you for free no questions asked. Its truly the best coverage you can get. At my store alone we give away 150 tires each month on average. Another nice thing about both warrentys is that it covers you on or off road.