Numbers Game: How To Understand The Information On Your Tire Sidewall

May 28, 2015
Story By
Harry Wagner

When we look at the side of a tire, our eyes are usually drawn to the lugs on the sidewall more than to the fine print found around the perimeter. The information found on the side of a tire can be very useful though - for everything from knowing how large it is, to whether it's a good choice for your application, to when it was made. Want to make sense of the information on your current tire’s sidewall or know what you are looking at when buying a set of new or used tires? Read on or click through to the section you're wanting to know more about.

1. CONSTRUCTION

2. WEATHER RATING

3. LOAD & SPEED

4. SIZING

5. MANUFACTURE DATE & DOT CODE

CONSTRUCTION

Radial Construction- Radial construction is the most common in modern tires, but bias ply tires are still offered for off-road use. As the name implies, radial tires use steel belts that travel around the radius of the tire carcass at a 90 degree angle to the center of the tire.

Bias Ply- Bias ply tires use nylon belts oriented at a 30-45 degree angle of the tread center line running all the way from bead to bead for the same construction and thickness on both the sidewall and the tread surface. This results in a very strong, puncture resistant sidewall. The drawback is that bias ply tires tend to be heavy, are stiffer than comparable radial tires, and can flat spot in cold weather and when sitting for extended periods of time.

Plies and Belts- Belts may be constructed of nylon, polyester, or steel. A radial tire allows the sidewall and the tread to function as two independent features of the tire. As a result, the ride quality and tread life tend to be better on a radial tire when compared to a bias ply tire.

Directional and Asymmetrical- Some tires are directional and/or asymmetrical, requiring them to be mounted in a specific orientation. A directional tire is one that has differing tread patterns fore and aft, while an asymmetric tire has different tread patterns across the face of the tire. Directional tires feature arrows on the sidewall that indicate what direction the tire should rotate when the vehicle is moving forward. Asymmetrical tires have the word "outside" labeled on the side of the tire that should face outward from the vehicle.

WEATHER RATING

M+S - M+S denotes a mud and snow tire, which may get you past the Department of Transportation road workers on a snowy road, but is a fairly vague designation when it comes to testing as this can be found on nearly all modern truck tires. Three Mountain Snowflake - The Three Peak Mountain Snowflake has more stringent requirements and lets you know this is a tire ready for snow and ice conditions, like the Nitto Exo Grappler pictured here. In order to meet this standard, the tire must attain a traction index equal to, or greater than 110 (compared to a reference tire which is rated 100) during traction tests specified by the ASTM on packed snow.

LOAD & SPEED

Load Range- Load range is used to indicate how much weight a tire can carry, the higher the letter, the more weight the tire can safely carry. A "Load Range C" rating indicates the tire has a 6-ply equivalent load carrying capacity. The tire may not actually have 6 plies, this rating comes from when bias ply tires were the standard. A "D" tire has an 8-ply rating, and an "E" a 10-ply rating. Note that maximum load rating values are given at maximum inflation pressures, lower pressures can hold less weight, but provide better ride quality if you are not carrying a lot of weight. Higher load ratings generally suggest a stronger tire for off-road use, with the tradeoff being that the stiffer tire might not conform as much to the terrain.

Load and Speed Ratings- 125/122 seen in the above tire sidewall refers to the load rating of the tire. 125 denotes that each tire can carry 3638 pounds at the maximum inflation pressure. The second number (122) refers to the load rating on a single tire used in a dual rear wheel application. “Q” refers to the tire’s speed rating. In this case, the tire is rated at 99 mph for continuous use. The higher the letter, the faster the tire is rated to travel. 10 PR refers to the ply rating of the tire. A 10-ply rated tire is the same as a Load Range E tire.

SIZING

Floatation Sizing- Floatation sizing is pretty easy to understand. The first number indicates the tire's diameter in inches (in this Nitto Terra Grappler G2, 35 inches tall). The next number (12.50) is the width, and the “R” that follows indicates that this is a radial tire. The last number (17) is the wheel diameter in inches. Like we said, pretty easy. Note that all tires tend to measure smaller than they are actually labeled, and the amount varies by model and manufacturer.

Metric Sizing- Metric sizing is more common these days than floatation sizing, and like the rest of the metric system can seem a bit foreign. In this example, the “LT” denotes this as a light truck tire, as opposed to a passenger car tire (which would be indicated by a “P”). “275” is the section width, in millimeters. The larger the number, the wider the tire is. “70” is what is called the aspect ratio, which means that the sidewall is 70% of the section width (which in this case is our 275 number from earlier). So a higher aspect ratio results in a taller sidewall, but a wider section width, with the same aspect ratio also results in a physically taller sidewall. After that the sizing matches floatation sizing, “R” is radial construction and “18” is the rim diameter (in inches for some reason). To calculate a metric tire’s height in inches, use the following formula: Section Width x Aspect Ratio/2450 x 2 + Rim Diameter 2540 corresponds to the fact that there are 25.4 millimeters per inch. The "2" is because the sidewall is found on both the top and bottom sides of the rim. So carrying out these calculations for the tire used in our example, you'd get 275x70/2450 x 2 + 18 = 33.15". Confused yet?

MANUFACTURE DATE & DOT CODE

DOT Number and Date Code- The DOT Tire Identification Number is perhaps the most mystifying piece of information on your tire’s sidewall. DOT stands for Department of Transportation, that part is easy enough. This is followed by ten, eleven or twelve characters that can be used to identify the manufacturing location, tire size and manufacturer's specifications, along with the week and year the tire was manufactured. In this case, “73” is the plant code, “BN” is the tire size, and “P4F” is the tire type. This information is only really useful to us in the event there is a tire recall. The last four numbers are important though, particularly when buying used tires. “2814” is the month and year that this tire was manufactured. So this tire was made in the 28th week of 2014.