It is important that if you want to maintain your automobile's original ride and handling, make sure the aspect ratio of your new tires matches the ratio listed on your car's original tires. The ratio of a tire's height to it's width is called it's aspect ration. If looking at the tire, find the two numbers listed before the letter R. The numbers designate your tire's aspect ratio. For example a tire with the size designation P215/65R15 has an aspect ratio of 65 which means the tire is 65 percent higher than it is wide and as such you would want 65-series tires.