how to read tire size and numbers

How to Read Tire Sizes & Tire Numbers

If you ever notice that string of numbers and letters on the sidewall of your tire, it’s there to tell you all the information that you need for your tires. Let the team at Sonia’s CDJR break down the process on how to read tire sizes from those numbers along the side and how to check your tire pressure! If you have any questions, feel free to reach out in the meantime. 

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Guide to Tire Size Numbers

When you look at the sidewall of your tire, you should see a code that includes all of this information:

  • Type
  • Width
  • Aspect Ratio
  • Construction
  • Rim Diameter
  • Load Index
  • Speed Rating

But it might be hard to translate from P 225 / 70 R 16 91 S to choosing the right tires for your car. Going from this example:

  • Type: This is the type of tire, here it's "P" for "Passenger." You might also see "LT" for "Light Truck."
  • Width: This is how wide the tire is, here it's 225. If that seems large, it's because the width is measured in millimeters.
  • Aspect Ratio: This is a ratio of the tire's height (off the rim) to its width. Here, the tire height is 70% of its width.
  • Construction: You'll most likely see "R" for "radial" here. "D" or "B" or "-" indicates diagonal or bias ply tires. You might also see an "RF" here if you have run flat tires.
  • Rim Diameter: Here, 16. This is the rim size in inches that the tire will fit.
  • Load Index: This is a number indicating how much weight the tire can support when properly inflated, but it depends on a variety of factors. Unless you're concerned with GVWR, you likely won't need to worry about the load index.
  • Speed Rating: Most passenger cars will have an "S" here for a max speed of 112 mph, though you might see an "R" for a max speed of 106 mph. 

What about tire pressure? Shouldn't the tire list the pressure it needs to be at? While your tire should have a pressure rating on it, this is a maximum pressure rating. It is not an indication of what pressure your tires should be at while driving around the area. 

How to Check Your Tire Pressure

You'll need a tire pressure gauge to check your tire pressure, and then it's as simple as taking off the valve stem cap and applying the end of the gauge to the stem tightly (you shouldn't hear any air coming out). Now that you know the pressure of your tires, though, you'll need to check your owner's manual or driver's door jamb for the OEM-recommended tire pressure.

If you're above the recommended pressure, you might need to bleed some air from the tires, while if you're below, you'll want to get to an air pump and top them off. If you want to ensure even tire wear and maximize your tires' lifespan, keep their pressure at the OEM recommendations.

Learn More About Your Tires at Sonia’s CDJR

The code has all the information that you need, but if it is time to upgrade them in Orange, MA, find the right tires at Sonia’s CDJR. We can tell you more about tire upkeep, including the importance of tire rotations and more!