At some point soon I plan on upgrading to 4 wheel disc brakes....there is an unending supply of options to go with with rotors in all sizes. This discussion I want to limit to rotor size....there is a crazy obsession in the car modification world that bigger=better when it comes to brakes. I see new cars being made with 20+ inch wheels so they can fit "bigger brakes" but it seems to me its a case of a dog chasing its tail here....those cars need bigger brakes because they are so much heavier. We will take the example of a base 2015 v6 of 3500lbs compared to a base 1966 I6 of 2500lbs as an example...thats a 30-40% increase in weight. If we assume the base 2015 v6 has a rotor size of 12.6" both front and rear then theoretically for the same braking performance on a 1966 a 9" rotor would suffice(not that you could find a rotor that small). I know that of course only the surface area under the pads actually counts and not my quick and dirty math using the full rotor diameter and that the larger the rotor, the more surface area is under the pad. What I am really trying to determine here is the minimum rotor size I need to get comparable braking capability to a modern mustang without going bigger than I need. Of course calipers are going to factor into the equation...but I want to limit discussion to rotor size first, maybe find a formula that will help me make an intelligent choice.