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What do the numbers on my tire’s sidewall mean?

Numbers...lots of numbers

Considering how much people rely on their tires, they’re still one of the least-understood parts of a vehicle. One of the most mystifying parts, for many people, is the set of numbers and graphics on the tire’s sidewall. This information might seem like some arcane code, but it holds a lot of important details about a tire.

All it takes is a little insight and you’ll be able to find out some stuff about your tires that really might come in handy down the line.

Tire Markings-TireSize.LoadIndex.SpeedRating.UTQG.DOT

Tire Markings For Load Pressure and Weather

Industry standards and standardized info

Size

Typically, you’ll see size expressed as something like P235/55R18. All that means is:

P stands for P-Metric, the industry’s most common classification. The P also stands for Passenger, but P tires are common for cars, minivans, light trucks, crossovers (CUV’s), and SUVs. Purpose-built light truck tires will have an LT as part of this designation

235 the width of the tire from sidewall to sidewall, in millimeters, so in this case 235 mm or about 9.25 inches (aka “section width”)

55 aspect ratio (or profile height), which is the height of the sidewall versus the width of the tire, expressed as a percentage. In this instance, the 55 means the tire is 129.25 mm, which is 55% of the 235 mm of the tire’s width

R stands for Radial construction, the most common design in modern tires. A bias-ply specialty tire will have a D in this sequence rather than an R

18 this just refers to the wheel size, in this case 18 inches

For some off road/all terrain tires, you may see the size expressed as inches, such as 33 x 17, meaning it’s a 17” rim and the tire’s total height is 33” tall.

Load index

Not surprisingly, every tire can only handle so much weight before it fails and something goes “boom.” Decades ago, the rubber industry standardized the code for load ratings

The load rating is expressed as a two-digit number, with most car tires in the 80s-90s range and light truck tires rated from the 90s to the 120s.

Here's a chart:

SimpleTire Chart Load Index Pounds Chart

The load index is per-tire, so if your tires are rated at 93 and can handle 1,433 pounds each, your total load rating would be 5,732 pounds. If your vehicle weighs 4,000 lbs (typical), you can put in another 1,732 pounds of people and stuff without risking tire failure.

This isn’t a concern for a lot of people, but it’s pretty important information to know for SUVs and trucks, especially work vehicles that need to haul heavy materials.

SimpleTire Load Range Ply Max Load Carrying PSI

Speed rating

This is another one that was standardized years ago by the rubber industry, and it’s an alphabetical code that shows the maximum speed the tire was rated for, by the manufacturer. It’s usually after the load rating on most tires.

Here's a chart:

SimpleTire Chart for Speed Rating Maximum Speed

DOT Code

This one is the longest number anywhere on the sidewall; this 12-digit number conveys the factory where the tire was built, as well as the week and year of manufacture. A tire with 4515 at the end of the DOT code, then, would have been manufactured on the 45th week of 2015.

Date of manufacture is important because tires do have a shelf life; establishing when a tire was made can help in deciding whether it needs to be replaced, regardless of how many miles are on the tread.

UTQC

uniform tire quality grading

Treadwear grade

This is a pretty easy one -- it’s a three-digit number that reflects how long the tread is expected to last. A tire with a 300 treadwear grade will last three times as long as one with a 100 grade, so that’s pretty simple...but, it can be a little deceptive too. Manufacturers aren’t held to a strict standard on their treadwear calculations, so there’s a considerable amount of wiggle room on this (especially on the cheaper tires).

Traction grade

This one is determined through testing that involves dragging a tire across wet pavement and measuring G-forces and friction. It’s expressed as a letter grade; from best to worst, the ratings are AA, A, B and C.

SimpleTire Traction Grades Asphalt g-Force Concrete g-Force Chart

Temperature grade

Heat dissipation is an important consideration, since heat buildup is the enemy of any tire; it’s a big factor in even tread wear and tire longevity in general. The temperature grade is a reflection of the tire’s ability to dissipate heat. An A temperature rating is over 115 degrees, a B rating is 100-115 and a C rating is 85 or under.

SimpleTire Temperature Grades MPH Chart

M+S/3PMS

All season tires that meet DOT minimum requirements for traction on mud and snow will be stamped with “M+S” somewhere on the sidewall. All terrain or winter tires, on the other hand, will have a graphic of a snowflake superimposed over a triple mountain peak; known as Three Peak Mountain Snowflake or 3PMS tires, these meet an entirely different and more stringent set of standards for winter traction.

Now you know

You don’t need some sort of insider knowledge (okay, maybe a little insider knowledge) or advanced training in cryptography to decipher what these codes mean. Next time you go out to your car, kneel down and check out the numbers for yourself, paying particular attention to the tread and the age of your tires. It can help you out a lot the next time it’s time to buy tires from SimpleTire, especially if your vehicle might have used more than one tire/wheel size -- just put your tire size information into our search tool and we can take you right to the best options for your car.

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