The size and ratings are printed on the side of your tires. You should stay with the same size as the manufacturer installed when the car was built. The easiest way to tell if the size is wrong, is if you think the speedometer is wrong. A smaller installed tire will make the speedometer read faster than the car is actually travelling, a larger tire will read slower. http://www.rimsntires.com/specs.jsp is a website that will let you enter the car model and year and will display stock wheel and tire information.

The higher the load number the higher the load carrying range. For a normal passenger car you don’t need to worry about the load range. The speed chart shown below explains the speed symbol.