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

asked by on November 16, 2015

How can I determine my tire's size?

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.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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


Experienced Mechanic?

Earn up to $70/hr

What others are asking

How do I program MyKey?
The MyKey feature can be programmed to restrict certain driving functions. Functions like max speed max volume promote safe driving. To program MyKey. Press the Setup button until you see “Reset for MyKey Settings.” Press the Reset button. Press the...
Engine waterlogged
There is a possibility that your vehicle's engine took on water, or the electrical system has failed. In the event of a flood, it is a hard decision to make at times, but the vehicle must be shut off immediately....
Steering wheel hard to turn after alternator replacement
Have the system scanned to see what is causing the light to turn on first. The low voltage conditions might have caused the problem and would need diagnosis of the code to fix the issue. Your power steering pump (https://www.yourmechanic.com/article/symptoms-of-a-bad-or-failing-power-steering-pump)...
I have a 2011 Infiniti QX56 and I have power but the radio and nav system won't come on and the car will not turn over
Hi. Although you have enough power for the dash lights and the interior lights to come on there may not be enough power in the battery for the truck to start and power the radio and navigation. The battery has...
Black smoke coming from rear tires
Hello. This is very abnormal for your tires (https://www.yourmechanic.com/article/why-do-tires-wear-unevenly) and should be looked at by an expert very soon. It sounds like you may have something that is rubbing on the tire causing it to smoke. However, generally when a...
Car cut off while on highway. It turns over but won't start - 1996 Chevrolet Caprice
If the car is cranking and attempting to start, then your issue is with the fuel, air, or spark. You will want to start by spraying starter fluid in the air intake and see if it starts, or at least...
Very low fuel efficiency
Your mileage is low compared to the EPA rating. Your vehicle was sold with a 3 yr/36,000 mi basic, 5 yr/60,000 mi powertrain warranty. In the event that there is an engine or transmission problem that accounts for the low...
Car has no electrical power at all
A number of components or situations can be causing this. If the alternator is not charging and providing the electrical needs of the vehicle, the battery will be drained. Once it is dead, nothing in the car will work. Other...

Related articles

Why Do New Tires Have Rubber Hair on Them?
There There are little rubber hairs that you see on every new tire (https://www.goodyear.com/en-US/company/partner/yourmechanic?promoCode=YOURMECHANIC&utm_source=null&utm_medium=referral&partnerId=yourmechanic&partnerName=yourmechanic&retaggingLaunchUrl=%2F%2Fassets.adobedtm.com%2Flaunch-EN786b79de5697438cb1f8839bcd30487c.min.js&floodlightEnabled=true&liveChatEnabled=true&siteName=Goodyear+Tires). Technically, they are called vent spews, which gives away their purpose for being on the tire. Many people think these hairs play a role in noise...
How Can I Tell If My Tires Are in Good Shape?
Car Car tires only have a certain service life. A vehicle must always have tires that are suitable for normal driving conditions. Many people who live in colder climates have two sets of tires - one for winter and one...
How Do I Prepare My Car for the Rainy Season?
Rain Rain doesn’t typically require making major modifications to your car, but following a few precautions at the start of any rainy season can make driving easier and safer. Check your tire tread to detect wear: Try the penny test....