Skip the auto shop - Our mechanics make house calls

Q: How can I determine my tire's size?

asked by on

How can I determine my tire's size?

A: Most of us never think about what size our ...

Most of us never think about what size our tires are; it’s something that usually remains out of mind until getting ready to purchase tires, or thinking about changing tire size. The size of your tires can be easy to find if you know what you are looking for, and what you are reading. Knowing what the numbers on the sidewall of the tire mean can make you a more informed tire buyer, and keep you from getting ripped off.

You can either get the tire size from the placard sticker on the inside of the driver front door jamb, or simply read it off of the sidewall of the tire itself. The most common tire out there today is known as a P metric. The P stands for "passenger tire". The numbers after the P are what indicate the tire size. The first number is in relation to the tire’s width. This number indicates how wide the tire is from sidewall to sidewall in millimeters. The second number shown after the width is in relation to the height, but it’s a little bit different. This number is known as the "aspect ratio", which is a percentage of the height. To illustrate, if you have a tire that is 255 millimeters wide, and has an aspect ratio of fifty, then the height of the sidewall is 50% of the width of the tire, or 127.5 millimeters.

The last number you will see on the tire size marking is rim size. This number tells you what size wheel the tire will fit on. A tire with an 18 after the aspect ratio means that that tire will fit on an 18” wheel, and only an 18” wheel; you cannot fit a tire onto the wrong size wheel as it can cause serious problems. It’s a good idea to use the same size tires that the vehicle came with, for a variety of reasons. Also, keep in mind that there are other tire types out there, besides P metric, that have a little different information, although it is not too common.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Is there a model number, etc. on my car tire that can help me find its speed rating and load index (service description)?

Your tires are an essential part of your vehicle. They can affect it in so many different ways. Not only are they a component that we can expect to spend money on over time, but they also have a huge...

Q: Low gas mileage

Hello. The fuel mileage rating on most vehicles is a rating estimate of the fuel mileage you should get. The problem with this rating you see in the windshield is based on city or highway driving. Both of these ratings...

Q: How Can I Extend the Life of my Tires?

Tires can be a costly part of the car owning experience, especially if you happen to be driving a large truck or SUV. Learning to recognize the signs that your tires are in good health or not is the first...

Related articles

How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the P0052 code means This code is seen when the Engine Control Module (ECM) tries to control the...
Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...