What Does a Tire Size Look Like?
Look at the sidewall of any tire and you'll see something like 215/65R16 or 245/40R18. Every passenger vehicle tire uses this same format, and once you understand what each part means, you'll never be confused at the tire shop again.
Breaking Down Each Number
The First Number — Section Width
The first number (215 in our example) is the tire's section width measured in millimeters. This is the width of the tire from sidewall to sidewall when mounted on the correct rim. A wider tire gives more contact patch with the road, which can improve grip — but it also increases rolling resistance and can reduce fuel economy.
The Second Number — Aspect Ratio
After the slash, you'll find the aspect ratio (65 in our example). This is the sidewall height expressed as a percentage of the tire's width. So a 215/65 tire has a sidewall height of 65% of 215mm, which equals about 140mm. Lower aspect ratios mean shorter, stiffer sidewalls — common on performance cars. Higher ratios mean taller sidewalls that absorb bumps better.
The Letter "R" — Construction Type
The R stands for Radial, which describes how the internal cords of the tire are constructed. Nearly all modern passenger tires are radial construction. You may occasionally see a "D" for diagonal bias construction on older vehicles or certain off-road tires.
The Final Number — Rim Diameter
The last number (16) is the diameter of the wheel rim the tire is designed to fit, measured in inches. This must match your wheel exactly — a tire rated for a 16-inch rim will not fit on a 17-inch wheel.
Why Getting Your Tire Size Right Matters
Using the wrong tire size isn't just an inconvenience — it can affect your vehicle's safety and performance in several ways.
- Speedometer accuracy: A significantly different overall tire diameter will cause your speedometer to read incorrectly.
- ABS and traction control: These systems are calibrated for your factory tire size. A large deviation can cause them to function incorrectly.
- Clearance issues: A wider or taller tire may rub against your wheel well, suspension components, or brakes.
- Handling: Tires that are too wide or too narrow for your rims can cause unpredictable handling, especially in wet conditions.
How to Find Your Correct Tire Size
Your vehicle's recommended tire size is found in three places: on the sidewall of your current tires, on a sticker inside the driver's side door jamb, or in your owner's manual. Always use the manufacturer's recommended size unless you're intentionally upsizing with a proper fitment guide.
💡 Quick Tip: If you're comparing a new tire size to your current size, use our free Tire Size Comparison Tool to see exactly how the dimensions differ and what impact the change will have on your speedometer.
What About the Other Numbers on the Sidewall?
Beyond the size code, your tire sidewall also shows a load index (the maximum weight the tire can support) and a speed rating (the maximum sustained speed). For example, a tire marked 215/65R16 98H has a load index of 98 (about 1,653 lbs per tire) and an H speed rating (up to 130 mph). Always match or exceed your vehicle's required load and speed ratings when replacing tires.
Thinking about a different tire size? Compare dimensions side by side with our free tool.
Try Tire Size Comparison →This article is for informational purposes only. Always consult your vehicle's owner's manual and a qualified tire professional before changing tire sizes.