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Decoding Your Tires

Updated September 21, 2015

The US Department of Transportation (DOT) requires tire manufacturers to put certain codes on their tires so that consumers can better understand what they are buying and service personnel what they are selling.  That being said, very few people know what these codes actually represent, even seasoned mechanics, so here’s your opportunity to learn how to decode your car’s tires like a pro.

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The First Section

Let’s start with the most frequently used block of code.  Look for a series of letters and numbers that look something like this: 245/50R17 89H. These represent the most preliminary information about the tire and maybe all you need. Here’s what these numbers represent:

245 is the tire’s width in mm.

50 is a measure of the tire’s profile.  This number is a ratio. It is the height of its sidewall relative to its width. This number is often referred to as the tire’s series; a sporty tire with shorter sidewalls would be considered a lower-series tire. Off-road tires are considered higher-series.

R refers to the tires design which is here is Radial.  Years ago there used to be other tire designs (Bias-Ply and others) but radials are the most common tire today.

17 is the diameter, in inches, of the rim on which the tire fits.

89H is the service description. Separated from the main code by a space, it represents the tire’s load and speed ratings. This tire’s 89 load index represents 1,279 pounds (per tire), and the speed rating of H represents 130 mph. You can find out what ratings apply to your tire by visiting the US Department of Transportation website

The NHTSA Section

Next up on the sidewall is part of the National Highway Traffic Safety Administration’s uniform tire-quality grading system. The section may look like this: TREADWEAR 150 TRACTION A TEMPERATURE B – M+S It appears on all-season and summer tires, but not on snow tires, light-truck tires or spares.

Treadwear 150 – Roughly how long the tire a tire will last.  Theoretically, a tire rated 150 would have a 50 percent longer lifespan than a tire rated 100 if used in the same conditions — driver, vehicle and roads. Soft summer tires tend to have lower treadwear ratings than all-season tires.

Traction A – This rating refers to traction on wet roads and uses AA, A, B and C, with AA being the best traction. The traction measured is straight-line acceleration and braking. This is not a measure of cornering grip or performance on dry surfaces.

Temperature B – Uses letter grades A, B and C, with A representing the best resistance to heat buildup and C the least. The friction of a tire on pavement generates heat, and too much heat degrades high-speed performance and can accelerate aging and failure. C is the lowest permissible rating.

M+S stands for mud and snow, indicating that this is an all-season tire. It’s not the clearest of the specs, because there aren’t other designations for summer and winter tires: Summer tires simply lack the M+S.

 

 

The DOT Section

The last spec we would like to call your attention to is the Department of Transportation’s Tire Identification Number (TIN). It may look like this: DOT H25R YC24 4305. It contains the week and year of the tire’s manufacture, and thus its age.

H25R YC24 – These numbers are like a VIN number and aren’t of interest to consumers.

4305 – This number is important.  The tire above was manufactured in the 43rd week of 2005. Most automakers agree that 10 years is the maximum safe lifespan for any tire, including a spare that has never been used.

The Maximum Inflation Pressure

you look closely enough, you’ll probably find a maximum-pressure figure on a tire’s sidewall. This is NOT the recommended pressure for you to use; it’s the maximum rating for the tire! The recommended pressure to inflating your tires to is located on a sticker on your vehicle’s driver side door jamb. It will be less than the maximum tire pressure that is stated on the actual tire.
Source: Lynch Chevrolet of Mukwonago

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Chris Riley
About Chris Riley

I have been wrecking cars for as long as I've been driving them but I keep coming back for more. Two wheels or four, I'm all in. GearHeads.org gives me a chance to give something back to the automobile community.

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