Tires: Square inch of tire contact, road surface, actual size

Question
How do you determine what the square inch of contact a tire has with road surface?

Answer
Chris,

The actual size of the contact patch requires much more detailed info and only the only way to tell is actually load the tire and measure it. In order to do that, you have to have some ink or some other way to make an impression on a piece of paper you would put under the tire.