A tire size chart may look like a jumble of numbers and letters, but each character carries one specific piece of information. Once you know what those characters mean, you can read any tire sidewall in seconds and choose a replacement with confidence.
This guide breaks down a standard tire size step by step, explains the extra markings that usually appear beside it, and shows how to use a size chart to compare tires.
What a Tire Size Chart Shows
A tire size chart is a reference that lists tire dimensions in a standard format. The most common format is the metric size, which looks like this:
P225/60R16 98H
Read as a whole, that string tells you how wide the tire is, how tall its sidewall is, what size wheel it fits, how much weight it can carry, and how fast it can safely go. Here is the quick version:
- P – Tire type (passenger vehicle)
- 225 – Section width in millimeters
- 60 – Aspect ratio (sidewall height as a percentage of width)
- R – Radial construction
- 16 – Wheel diameter in inches
- 98 – Load index
- H – Speed rating
Everything after the wheel diameter is listed in the chart as well, but it describes performance rather than physical size.
How to Read the Four Main Dimensions
1. Tire Type Letter
A letter at the front shows what kind of vehicle the tire was designed for. P means passenger car. LT means light truck. Some tires have no letter at all, which means they follow the same metric format without a vehicle-type prefix.
2. Section Width
The first number is the width of the tire in millimeters, measured from sidewall to sidewall at the widest point. A 225 tire is 225 millimeters wide, which is about 8.9 inches. A larger number means a wider tire.
3. Aspect Ratio
The number after the slash is the aspect ratio, also called the profile. It is not a measurement on its own. It is a percentage of the section width.
For a 225/60 tire, the sidewall height is 60 percent of 225 millimeters, or 135 millimeters (about 5.3 inches). A lower aspect ratio, such as 45, means a shorter sidewall and a sportier look. A higher ratio, such as 75, means a taller sidewall and a softer ride.
4. Construction and Wheel Diameter
The letter R stands for radial construction, which is standard on nearly all modern tires. It does not mean radius. The number that follows is the wheel diameter in inches, so a 16 in this position fits a 16-inch wheel.
Load Index and Speed Rating
The last two characters describe how much weight and speed the tire can handle.
The load index is a code that maps to a maximum carrying capacity per tire. Higher numbers mean more capacity.
- 91 – about 1,356 lb (615 kg)
- 94 – about 1,477 lb (670 kg)
- 98 – about 1,653 lb (750 kg)
- 100 – about 1,764 lb (800 kg)
The speed rating is a letter that shows the maximum sustained speed the tire is designed for. Common ratings include S (112 mph), T (118 mph), H (130 mph), V (149 mph), and W (168 mph). Always match or exceed the rating listed on your vehicle’s tire placard.
Other Markings You Will See on the Sidewall
- M+S or M/S – Rated for mud and snow conditions.
- Three-peak mountain snowflake symbol – Meets stricter winter traction standards.
- DOT code – A production code; the last four digits show the week and year the tire was made.
- Max load and max pressure – The tire’s own limits, not the vehicle’s recommended settings.
- Treadwear, traction, and temperature grades – Uniform quality ratings found on most passenger tires.
For everyday driving, the tire placard on the driver’s door jamb, fuel door, or in the owner’s manual lists the size and pressure the vehicle was designed for. That is the starting point for any replacement.
Reading a Tire Size Chart in Practice
A size chart usually lists several sizes in rows, with the width, sidewall height, and overall diameter in separate columns. Overall diameter matters because it affects your speedometer, odometer, and clearance inside the wheel well.
You can calculate it with this formula:
Overall diameter = (section width × aspect ratio ÷ 25.4 × 2) + wheel diameter
Here is how a few common sizes compare:
| Tire Size | Section Width | Sidewall Height | Wheel Size | Overall Diameter |
|---|---|---|---|---|
| 205/55R16 | 205 mm | 113 mm | 16 in | 24.9 in |
| 225/60R16 | 225 mm | 135 mm | 16 in | 26.6 in |
| 235/55R17 | 235 mm | 129 mm | 17 in | 27.2 in |
| 245/75R16 | 245 mm | 184 mm | 16 in | 30.5 in |
Notice that a smaller wheel can still produce a larger overall diameter. This is why comparing only the wheel size is not enough.
Metric, Flotation, and Light Truck Formats
Not every tire uses the metric format. You may also see:
- Flotation sizes, such as 33×12.50R15. These list overall diameter in inches first, section width in inches second, and wheel diameter last. They are common on trucks and off-road vehicles.
- Light truck metric sizes, such as LT245/75R16, which follow the same layout as passenger sizes but with a heavier load range.
- Load range letters, such as C, D, or E, which indicate how much air pressure and weight a tire is built to handle.
Different formats still describe the same three basics: width, height, and wheel size.
Tips for Choosing the Right Size
- Start with the size on your vehicle’s tire placard.
- Keep all four tires the same size unless the vehicle manufacturer says otherwise.
- Avoid mixing sizes or types on the same axle.
- If you change sizes, keep the overall diameter within about 3 percent of the original to protect speedometer accuracy and avoid rubbing.
- Check the load index and speed rating, not just the physical dimensions.
- Confirm the tire’s production date, since older tires may need replacing even with good tread.
Common Mistakes to Avoid
- Reading the aspect ratio as a direct measurement instead of a percentage.
- Assuming the R stands for radius.
- Using the max pressure printed on the sidewall as the everyday inflation setting.
- Buying a wider tire without checking wheel width and clearance.
- Ignoring the load index when switching to a different size.
Conclusion
A tire size chart is easier to read than it looks. The first number is width in millimeters, the second is sidewall height as a percentage of that width, the letter shows radial construction, and the number after it is the wheel diameter in inches. The final two characters, the load index and speed rating, tell you how much weight and speed the tire can handle.
When you need a replacement, match the size and ratings on your vehicle’s tire placard first, then use a chart to compare overall diameter if you are considering a different size. That simple habit keeps your speedometer accurate and your vehicle handling as intended.
For more practical guides on vehicle care, home maintenance, and everyday how-tos, explore the other articles on our site.