A Contact patch is the term applied to the portion of a vehicle's tire that is in actual contact with the road surface. The shape of a tire's contact patch can have a great effect on the handling of the vehicle to which it is fitted. Specifically, for the type of wide tire fitted to many modern performance cars, a contact patch that is wider than it is long will increase the tendency for the vehicle to 'tramline' or follow uneven road contours. Furthermore in front wheel drive cars, the offset between the centroid of the contact patch and the point about which the wheel steers can lead to a condition known as torque steer.
With normal street tires on an automobile the contact patch will remain uniform across the tread of the tire. If the tire is over-inflated the tire will tend to bulge in the center of the tread which will lift the edges off the pavement. This can decrease the handling performance of the vehicle and also decrease the life of the tire. Prolonged use of a tire which is over-inflated will cause the tread in the center to wear away faster than the tread on the edges.
An under-inflated tire can have negative effects as well. In this case the center of the tread will not make as much contact with the road surface and the edges of the tread will wear down faster because the sidewalls of the tire will push the edges into the pavement.
One method of checking for proper inflation is to find a long stretch of pavement such as an empty parking lot and then draw a line across the tread with chalk. Then simply drive straight across the parking lot. If the entire line of chalk has rubbed off, the tire is properly inflated. If the center of the line is rubbed off but the ends are still present, the tire is over-inflated. On the other hand, if the line is rubbed off at the ends but is still present in the center of the tread, the tire is under-inflated.