People who don’t speak the lingo can be absolutely mystified by some of the terms that come out of a mechanically inclined person’s mouth. But even the specialized vocabulary of the gear head often falls short when it comes to talking about the rubber. Car truck tires have their own jargon and terminology, and if you are able to decipher the coded numbers and letters found on their sides you will be one step ahead of most other people.
Hopefully, these are the only bits of your vehicle which come into contact with the ground. But because of this contact, you need to understand the limitations and capabilities of what’s on your wheels. You might assume that the majority of your ride’s functions are mostly dependent on your engine and drive train, but everything from acceleration to braking is primarily the responsibility of the rubber.
Using a P 245/55R18 90W tire as our example, let’s crack the code:
The General Use
P 245/55R18 90Z
The initial letter in the code, in our case a “P” denotes the basic intended use of the product. There are two broad categories and they are designated P and LT. Those with a P class are meant to be used on normal, everyday passenger vehicles. The letters LT denote a tougher construction, meant exclusively for haulage, transport, and larger vehicles.
The Width Across
P 245/55R18 90T
Measured in millimeters, this number indicates the width of the tire from sidewall to sidewall, excluding any lettering or designs. In this example, the tire is 245mm (9.65″) wide.
Aspect Ratio
P 245/55R18 90Z
This number is a measure of the aspect ratio. Derived by examining the relationship between the width and the height together, it is expressed as a percentage of the section width. Our test code gives us an aspect ratio of fifty five. This means that the sidewall height is equal to 55% of the section width, which works out to a little more than five and a quarter inches.
Interior Build
P 245/55R18 90Z
This letter indicates the internal construction of the tire, which you’ll generally find in two different varieties:
If there is an “R” in the code, as in the example, then we are looking at a radial construction. This means that the substrata of rubber, the plies, have been laid so that they run at a ninety degree angle to the central line. This diminishes rolling resistance while increasing flexibility at the same time.
If there is a letter “D” instead, then you have a bias ply in front of you. Similar to radial construction, the plies are instead arranged at an oblique angle to the center line. This is usually done at nearly a forty five degree angle, but can be as little as thirty two degrees. These are more appropriate for high aspect ratio and other special requirements.
The Wheel’s Diameter
P 245/55R18 90T
Now we arrive at the rim size. This measurement tells you the diameter of the rim or wheel that will fit into the rubber ring. It is expressed in inches, and is usually a whole number. Our rim size according to our imaginary code is 18, giving us an appropriate wheel diameter of eighteen inches.
Weight Supporting
P 245/55R18 90T
The load index refers to the amount of weight a tire can support. As the load index increases, the maximum weight increases accordingly. Typical passenger vehicle tires have a load index of 70 to 110 (761-2,337 lbs.). With a load index of 90, this tire can support 1,323 lbs.
Speed Rating
P 245/55R18 90T
For high speed highway travel, a universal way to designate the appropriateness of certain models for certain speeds was developed. This allows drivers to choose the most fitting products for their habitual driving styles. The letter range goes from M to Z, and each has a corresponding maximum safe speed. A “Z” speed rating, as is the case with our example, means that things should be fine at speeds in excess of 149 miles per hour.
Here’s a rundown of all tire speed ratings:
M rating: 81 mph
87 miles per hour (140 kph): N
P: 93 mph or 150 km/h
Q rating: 99 mph
R: 106 mph or 170 km/h
112 miles per hour (180 kph): S
118 miles per hour (190 kph): T
U: 124 mph or 200 km/h
H: 130 mph or 210 km/h
V rating: 149 mph
Faster than 149 miles per hour (240 kph): Z
168 miles per hour (270 kph): W
Y rating: 186 mph
Note that some tires include the Z speed rating within the size designation (example: P 255/50ZR18 W). Prior to 1991, Z was the highest speed rating given to tires, indicating that they had been tested to exceed 149 mph. Manufacturers commonly add this designation in addition to W or Y ratings.
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