At some point in your life, if you own a vehicle like a car, you are going to have to buy tires for it. You can't just buy any old tire and slap it on there; the tire must fit the wheel (BTW "wheel" refers to the entire assembly on to which your tire is fitted; "rims" are the outer edges of the wheel, where the tire seals to the wheel; the center part between the rims is called the "barrel"), and, of course, wheel sizes vary depending on the make and model of the vehicle.
How does one know what size of tire fits the wheels of their vehicle? In the 1970s, as part of safety standardization, vehicle manufacturers had to start displaying tire size information on a sticker inside the driver's side door jamb. The sticker will look like this:
This sticker is from my 2008 Saturn Astra XE. You can see sizes, a code for the speed ratings, and cold tire pressures (remember, as you drive a car the tires heat up and pressure increases) for the front tires, rear tires, and the spare tire. Let's decipher the codes so that you know what you are looking at.
Let's look at the code in the "original size" column for the front tires (same as rear in my case). The code is P205/55R16. The "T" toward the end of the column indicates the required speed rating of the tire. Here are what each of the parts mean:
| Code | Description |
| P | Passenger Tire (other options are LT for light truck; ST for special trailer; C for commercial truck; T for temporary [note the spare tire size begins with T]; there may be others for different specialty tires) |
| 205 | This is the tire width in millimeters, when looking at the tire head-on. It is basically the part with the tread on it. |
| 55 | This is the aspect ratio of the tire, which is essentially the height of the sidewall of the tire. Since it is a ratio, it should be read as a percentage of the width (imagine the tire cut and laid out flat so that the entire width includes the sidewalls and the tread part). So the aspect ratio of my tire is 55%. That means the height of my sidewall is 205mm x .55 = 112.75mm (or 4.44 inches). |
| R | This means "radial." If you see a "D," that stands for "diagonal" and means that you have "bias ply" tires. Radial tires are safer and preferred for modern vehicles. They also offer a smoother ride than bias ply. Learn more here. |
| 16 | This is the wheel diameter in inches. It's the distance between the two bead seat areas (rims). |
| T | This is the speed rating of the tires that are expected to be put on this vehicle. "T" means the tire is rated for speeds up to 118 MPH (which my car is not actually able to do). Note that the speed rating is "M" on my spare tire. That means it's rated to speeds up to 80 MPH. However, that does NOT mean that one should drive that fast on the little "donut" spare tire. Typically, your vehicle's owner's manual will tell you to avoid going much over 55 or 60 MPH on the spare. Check the owner's manual that is specific to your vehicle. |
Even if you don't remember how to decode the tire size, etc., when you need a new tire, at least you will know what information to give the tire shop if you're ordering your tires in advance of the service. When I call for a tire, I typically say something like, "Yes, hi. I have a 2008 Saturn Astra, and I need new tires. I'm looking for a P - 205 - 55 - R - 16. Do you have those in stock?" Of course the shop, if it is reputable, will enter your vehicle information into their system (especially the VIN) and it will tell them what size of tire you need.
I hope this post is helpful for you! Now all you need to do is figure out how to pay for your tires . . . they're not cheap! Good luck!