How to Read the Numbers and Letters on Car Tires
April 6, 2012 at 2:00 am Chad Upton 7 comments
By Chad Upton | Editor
The weather is starting to change in many parts of the world so this may be when you start to look at new tires for your car.
Tire specs can be confusing, but it’s just like replacing a vehicle fuse, light bulb or most other car parts — you look at the model number or specifications of the current one and you try to find another one just like it. The confusing part about tires is that you have more options; slightly different sizes, treads and styles are likely compatible with your wheels.
Understanding what the numbers mean will help you make the best decision about new tires.
Because the tread and maintenance of your tires is so important to your safety, fuel economy, and performance, it’s very important that you buy tires that are suitable for your climate and performance needs.
Just so there is no confusion, the word “tire” refers to the rubber that touches the ground. The tire is stretched around the metal “wheel” or “rim”. Some of this confusion comes from the phrase, “changing a tire.” When you change a tire at the side of the road, you typically change a wheel (that has a tire wrapped around it).
The manufacturer of your car will provide a recommended tire size for your car. It can usually be found in the manual or on the inside of the driver’s door frame (the suggested tire pressure is also listed here). You can also just look at one of the tires on your car, it will usually have all of the specifications on the side wall of the tire. It should look something like this: P215/45R17. If none of these options help, TireRack.com has a tool that will let you enter your car make/model/year and it will tell you which tires and wheels will fit on your car.
The first letter or pair of letters in the tire size (P215/45R17) tells you if the tire is meant for a Passenger car, Truck, Light Truck, Commercial vehicle or a couple other special cases.
The first set of numbers (P215/45R17) indicates the section width of the tire in millimeters. Without getting too technical, this is basically the width of the tire.
The next number is the sidewall aspect ratio. It represents the height of the tire — the distance between the outer edge of the wheel and the tread of the tire. This distance is measured as a percentage of the section width. In the example set (P215/45R17), the sidewall height is 45% of the tire width. Since the width was 215 mm, 45% of that is 97 mm.
If you’re replacing tires and you want to mount them on your existing wheels/rims then the wheel diameter and the section width need to match your rim. The wheel diameter is usually the last number in the tire size. In our example (P215/45R17), the last number is 17, which means the wheel diameter is 17 inches.
You may also notice the “Z” in the photo above. This is a speed rating. It’s not found on most tires, unless they are Z-rated. Z-rated means the tire is designed for faster speeds — they are generally harder tires which handle better at high speeds while “slower” tires are softer and provide a more cushioned ride.
Since we’re talking about tires, make sure you know what the tire pressure warning light looks like. About 33% of drivers don’t know what it looks like and it could save your life.
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Sources: TireRack.com 1, NHTSA, How Stuff Works, TireRack.com 2, TunerTools.com
Entry filed under: Automotive. Tags: diameter, rating, rim, size, speed, tire, tyre, Uniform Tire Quality Grading, UTQG.
Broken Secrets 
1.
automobiles105 | May 6, 2012 at 10:45 pm
Wow! This is a nice info about the numbers and the letters of the tires.
2.
Rich | May 10, 2012 at 9:21 am
I always knew hat th numbers are but the letters always confused me.
3.
Chris Evans | May 14, 2012 at 9:49 am
Maybe thats where I’m going wrong…. my diesel Audi A6 litereally eats front tyres. From now on I’m buying higher speed rated tyres so that I stop going through them so quickly. Thanks Chris
4.
Howard Cherry | August 4, 2012 at 12:19 pm
I would like to know if the tire has a code for when it has made I buy used tire and like to know how old they are thank s Howard
5.
Chad Upton | August 4, 2012 at 6:52 pm
Here’s an article that may help you answer this:
http://www.edmunds.com/car-care/how-old-and-dangerous-are-your-tires.html
6.
UK Mortgages | November 5, 2012 at 4:59 pm
Hi everyone, I’d like to know if the tread pattern has much of an impact in wet road conditions. I recently bought some Toyo tires that had wavy lines that filtered out to the edges. They were excellent tires and gripped well. I now need replacements and was thinking of some “AVON ZR5’s”. They have the same speed rating as the “Toyo’s” but the tread is in straight lines instead – does this mean they are not as good?
7.
Bernie | November 8, 2012 at 7:07 pm
is it ok to change the tire size on all four of my tires for my 2007 hyundai santa fe ( I have a set of 4 winter tires on rims with a different size)…will it cause safety probs or cause other probs with the car?