The five most popular tire sizes for passenger vehicles are P215/60R16, P225/50R17, P265/70R17, P195/60R15 and P215/55R17, according to Modern Tire Dealer. For light trucks, TireGuide.com indicates the most common size is LT245/75R17.
In order to interpret the meaning of the letters and numbers in a tire size, one must break it down into five sections. The P or LT in the beginning stands for Passenger or Light Truck. The next three numbers signify the width of the tire section in millimeters from sidewall to sidewall. The two numbers after the slash indicate the height of the sidewall as a percentage of the section width. The R stands for radial tire construction. The last two numbers represent the wheel diameter in inches.