Truck Articles

Many people are amazed at the crazy and wild sounding vocabulary which is thrown around by those who work in machine shops or other mechanical businesses. Even the most seasoned gear head may find that their mastery of this language suddenly seems incomplete when it comes to the mysterious letters and numbers written on car truck tires.

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.

We will being with an imaginary code. Let’s call it P 245/55R18 90T

The General Use

P 245/55R18 90W

For nearly all varieties, the first couple of letters denotes whether it is recommended for normal or heavy use. The two main divisions are “P” and “LT” with the former being designed for use on passenger and other commuter vehicles and the latter reserved for the more heavy duty vehicles like cargo vans and bigger pickups. Nearly all regular road vehicles will have P designations.

The Width Across

P 245/55R18 90Z

The next number is a measurement of the lateral width. Not counting any width which may be the result of designs or lettering, this is reported in millimeters. Our test code here reveals a width of 254mm, which is equivalent to a little over nine and a half inches.

Aspect Ratio

P 245/55R18 90W

Aspect Ratio is the height of the sidewall, as a percentage of the section width. In this example, the number 55 tells us that the height of the sidewall is 55 percent of the section width. To determine the height, simply multiply 245mm by 55 percent (.55). The tire’s sidewall is 134.75mm (5.30″) tall.

Internal Construction

P 245/55R18 90Z

The next letter tells you about the internal construction. There are two basic kinds.

Almost all rubber on the road is part of a radial. Ninety eight percent of manufacturing goes to making them, and they have the letter “R” in their codes. Radials are constructed in such a way as to minimize rolling resistance while simultaneously increasing flexibility through the use of perpendicular ply folding.

If the code has a letter “D” in it, then you are looking at a bias ply. These have an internal construction which is arranged along a diagonal from the center line, usually at a 32 to 45 degree angle. This is a very popular way to build high performance and off road models which need high aspect ratios.

The Diameter of the Wheel

P 245/55R18 90Z

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 Bearing

P 245/55R18 90W

Moving to the next digits, we get the load rating. This is not a direct measure, and you will need to consult a table of correspondences. Suffice it to say that as this number goes up, so too does the permissible load. Most vehicles use ratings between 70 and 110. Our code says 90, and that means a load capacity of 1,323 lbs.

Velocity Allowance

P 245/55R18 90W

The last letter represents a measure of speed testing which has been performed. It came about as a way for drivers to easily find the best rubber for their driving, especially if they spent a lot of time at high speeds. This rating runs from M to Z and tells you what the fastest safe rate of speed for the construction is. Ours is rated T, so we can safely travel up to 118 miles per hour.

The full list of ratings looks like this:

M: 81mph or 130 km/h

N: 87 mph or 140 km/h

93 miles per hour (150 kph): P

Q rating: 99 mph

R rating: 106 mph

S: 112 mph or 180 km/h

T rating: 118 mph

U rating: 124 mph

H rating: 130 mph

149 miles per hour (240 kph): V

Z: in excess of 149 mph or 240 km/h

W rating: 168 mph

Y: 186 mph or 300 km/h

Before 1991, the Z class speed rating was the highest that car truck tires could achieve. Since then there have been further designations, which explains the overlap between the Z class and a couple of the others. The Z class is often shown in the code along with the higher W or Y ratings.

We have Every thing you wanted to know about tires all right here just for you. How well do you know tires will never be a question you will stutter on again!

Apr082011

Tags: car tires, tires, Trucks

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