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・Save up to 30%

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: SUV rear end has roaring, grinding noises

Hello. The way to fix this problem is to replace the rear differential with a new one. You could also have the existing differential rebuilt in order to stop the problem. Another option would be to replace the damaged differential...

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 Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...