When owning a performance car, every bit of horsepower counts. That said, over time, wear and tear takes its toll on performance and some of those horses run away. Of course, it mainly depends on how the engine has been treated, both in terms of driving and maintenance. Mileage also takes its toll on the mechanical components of a car. But just how much power (if any) is lost over time? This tired Mustang Shelby GT500 answers that.