Passenger Tire Nomenclature
There are four systems used to identify passenger tire size nomenclature: Numeric, Alpha Numeric, European Metric and the P-Metric system. Alpha Numeric is basically a dinosaur. The most commonly used today are P-Metric and European-Metric. For example let’s look at a P235/60R16 89T
P: P is the most common and it means Passenger. LT stands for Light Truck application. A LT tire has a sturdier construction than a P tire, and it can carry a greater load.
235: The width of the tire, from sidewall to sidewall, in millimeters. A 235 is 235 millimeters, or 23.5 centimeters, wide. The tread width is actually somewhat smaller than the tire’s actual width.
60: Aspect Ratio. This is the sidewall’s height from the inside diameter to the outside diameter. It is expressed as a percentage of the width. A 235/60 is 60 percent as tall as it is wide, making it approximately 141 millimeters tall. The smaller the aspect ratio the firmer the tire is. Smaller aspect ratios are designed for performance tires used on sports cars.
R: The tire’s type of construction. R stands for Radial, and all new cars and light trucks today use radials. You may also see B in this spot for Bias Belted Ply, and that means it was created before the mid 70’s. Or there could be a D for Diagonal Bias.
16: Rim diameter in inches. The inner diameter of the tire matches the diameter of the rim.
89: Load Rating. Load ratings range from 0 to 279 and each has a corresponding weight associated with it. If you intend to tow a heavy boat or trailer, you should buy tires with a higher load rating. This will require some math to figure out the vehicle weight, the trailer weight, and the weight of a load you may carry inside the vehicle while towing. Add it all up, divide by four, and make sure your replacement tires have a corresponding load rating. T: Speed Rating. All passenger tires have a speed rating expressed as a letter. Those letters and the corresponding speeds the tires are capable of are included in this chart:
S: 112 mph
T: 118 mph
U: 124 mph
H: 130 mph
V: 149 mph
Y: 186 mph
Z: over 186 mph
A Speed Rating is really an indication of a tire’s ability to dissipate heat to avoid a blowout. Higher speeds mean greater heat buildup. Tires with higher speed ratings are constructed to handle heat better. In general, they also ride harder than tires with lower speed ratings. S-, T-, and U-rated tires are considered regular passenger-car tires. H- and V-ratings are reserved for touring tires, which are generally found on sport sedans.
The Z rating is for performance tires found on sports cars. M+S: Mud and Snow. This is an all-season tire. A summer performance tire would have no designation here.