How to Read Tire Sizes: Complete Guide with Examples
Tire size is a standardized code printed on the sidewall of every tire that describes its dimensions, construction type, and performance capabilities. A tire marked P225/65R17 98H tells you the tire is 225 mm wide, has a sidewall height that is 65% of the width, uses radial construction, fits a 17-inch wheel, supports 1,653 pounds per tire, and is rated for speeds up to 130 mph.
Quick Answer
- 1. First 3 digits (225) = tire width in millimeters from sidewall to sidewall.
- 2. Two digits after slash (65) = aspect ratio: sidewall height is 65% of the width.
- 3. R = radial construction, used in over 98% of modern passenger tires.
- 4. Last 2 digits (17) = wheel diameter in inches. Replacement tires must stay within 3% of original diameter.
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Compare Tire Sizes FreeBreaking Down the Tire Size Code
Let us decode the tire size P225/65R17 98Hpiece by piece. Each segment tells you something different about the tire's dimensions and performance.
P: Tire Type
The letter prefix indicates what type of vehicle the tire is designed for:
- P = Passenger vehicle (cars, minivans, small SUVs, light crossovers)
- LT = Light Truck (pickup trucks, heavy SUVs, vehicles used for towing)
- ST = Special Trailer (trailer tires only, never used on driven axles)
- No letter = Euro-metric sizing, functionally similar to P-metric but built to slightly different standards
225: Tire Width in Millimeters
The first three-digit number is the tire's section width, measured from the widest point of one sidewall to the widest point of the other sidewall, in millimeters. A 225 mm tire is approximately 8.9 inches wide. Common passenger vehicle tire widths range from 185 mm (compact cars) to 315 mm (high-performance sports cars).
Wider tires provide more contact with the road, which generally improves grip and handling. However, wider tires also create more rolling resistance, which can reduce fuel economy and increase road noise.
65: Aspect Ratio
The aspect ratio is the height of the tire's sidewall expressed as a percentage of the tire's width. An aspect ratio of 65 means the sidewall height is 65% of 225 mm, which equals 146.25 mm (about 5.76 inches).
Lower aspect ratios (like 35 or 40) mean shorter, stiffer sidewalls. These "low-profile" tires offer sharper handling and a sportier look but produce a firmer ride and are more vulnerable to pothole damage. Higher aspect ratios (65, 70, 75) mean taller sidewalls that absorb road imperfections better, providing a smoother, more comfortable ride.
R: Construction Type
The letter R indicates radial construction, where the internal ply cords run perpendicular to the direction of travel. Over 98% of all tires sold today are radial. The alternatives are:
- B = Bias-belt (cords crisscross at angles, mainly for trailers)
- D = Diagonal/bias-ply (older construction, rarely used on modern vehicles)
Radial tires offer better fuel economy, more even tread wear, longer life, improved handling, and a smoother ride compared to bias-ply tires.
17: Wheel Diameter in Inches
The final number is the diameter of the wheel (rim) that the tire is designed to fit, measured in inches. This tire requires a 17-inch wheel. Common wheel diameters for passenger vehicles range from 14 inches (small economy cars) to 22 inches (large SUVs and trucks). Performance vehicles sometimes use wheels up to 24 inches.
Understanding Load Index and Speed Rating
After the tire size, you will see a number and a letter, like 98H. These indicate the tire's load-carrying capacity and maximum speed rating.
Load Index
The load index is a numerical code that corresponds to the maximum weight the tire can support when properly inflated. It is not a direct measurement in pounds. Here are common load index values:
| Load Index | Max Weight (lbs) |
|---|---|
| 85 | 1,135 |
| 90 | 1,323 |
| 95 | 1,521 |
| 98 | 1,653 |
| 100 | 1,764 |
| 105 | 2,039 |
| 110 | 2,337 |
Speed Rating
The speed rating letter indicates the maximum sustained speed the tire can safely handle:
| Rating | Max Speed (mph) | Common Use |
|---|---|---|
| S | 112 | Family sedans, minivans |
| T | 118 | Family sedans, minivans |
| H | 130 | Sport sedans, coupes |
| V | 149 | Sports cars, performance sedans |
| W | 168 | Exotic sports cars |
| Y | 186 | Supercars |
Always replace tires with the same or higher speed rating and load index as the original equipment. Downgrading either can compromise vehicle safety.
Overall Tire Dimensions: Diameter and Circumference
The tire size code gives you enough information to calculate the overall tire diameter and circumference, which matter for speedometer accuracy and vehicle clearance.
For a P225/65R17 tire:
- Sidewall height: 225 mm x 0.65 = 146.25 mm = 5.76 inches
- Overall diameter: (2 x 5.76) + 17 = 28.5 inches
- Circumference: 28.5 x 3.14159 = 89.5 inches
- Revolutions per mile: 63,360 / 89.5 = 708 revolutions
These numbers are critical when considering alternative tire sizes. Our tire size calculator computes these automatically and shows the difference between your current and proposed tires.
Common Tire Sizes by Vehicle Type
| Vehicle Type | Common Sizes | Example Vehicles |
|---|---|---|
| Compact cars | 195/65R15, 205/55R16 | Honda Civic, Toyota Corolla |
| Mid-size sedans | 215/55R17, 225/50R17 | Toyota Camry, Honda Accord |
| Compact SUVs | 225/65R17, 235/55R18 | Toyota RAV4, Honda CR-V |
| Full-size SUVs | 265/70R17, 275/55R20 | Ford Explorer, Chevy Tahoe |
| Pickup trucks | 275/65R18, 285/70R17 | Ford F-150, Ram 1500 |
What Happens If You Change Tire Size
Speedometer Accuracy
Your speedometer is calibrated for the original tire diameter. If you install larger tires, your actual speed will be higher than what the speedometer shows. Smaller tires have the opposite effect. A 3% change in tire diameter produces approximately a 3% speedometer error.
Fuel Economy
Wider tires increase rolling resistance and can reduce fuel economy by 1 to 3%. Taller tires (larger diameter) lower engine RPM at highway speeds, which can slightly improve fuel economy but may reduce acceleration.
Vehicle Handling
Changing tire size affects cornering grip, braking distance, and ride comfort. Lower-profile tires improve cornering precision but ride harder. Higher-profile tires absorb bumps better but may feel less responsive in turns.
The Bottom Line
Reading tire sizes is simpler than it looks once you understand the code. The width is in millimeters, the aspect ratio is a percentage, R means radial, and the last number is the wheel diameter in inches. When shopping for replacement tires, always match the load index and speed rating of your originals, and stay within 3% of the overall diameter to maintain speedometer accuracy and vehicle safety.
Use our free tire size calculator to compare any two tire sizes and see the exact differences in diameter, circumference, speedometer reading, and sidewall height.
Frequently Asked Questions
What does the P in P225/65R17 mean?
The P stands for P-metric, indicating the tire was designed to the Tire and Rim Association standards used for passenger vehicles in the United States. If there is no letter prefix, the tire follows European Tire and Rim Technical Organisation (ETRTO) standards, also called metric sizing. An LT prefix means light truck, designed for vehicles that carry heavier loads or tow trailers. The performance characteristics are slightly different, so you should not mix P-metric and LT tires on the same vehicle.
Can I use a different tire size than what my car recommends?
You can change tire sizes within limits. The general rule is that replacement tires should be within 3% of the overall diameter of the original tires. Going outside this range can affect your speedometer accuracy, anti-lock braking system, traction control, and vehicle handling. It can also void your vehicle warranty. Always check your owner's manual or the tire placard on the driver's door jamb for approved alternative sizes before making a change.
What does the R in tire size mean?
The R stands for radial construction, which means the tire's internal ply cords radiate outward from the center of the tire at 90 degrees to the direction of travel. Virtually all modern passenger vehicle tires (over 98% of those sold today) are radial construction. The older alternative, bias-ply (indicated by B or D), has cords that crisscross at angles and is now mainly used on trailers and some specialty vehicles. Radial tires offer better fuel economy, longer tread life, and improved handling.
How do I find my current tire size?
There are three places to find your tire size: (1) On the tire sidewall itself, printed in raised text following the format like P225/65R17 98H. (2) On the tire information placard, usually located on the inside of the driver's side door jamb. (3) In your vehicle's owner's manual. The door jamb placard also shows the recommended tire pressure for your specific vehicle, which may differ from the maximum pressure printed on the tire sidewall.
What do the numbers after the tire size mean (like 98H)?
The number after the tire size is the load index, and the letter is the speed rating. In 98H, the 98 means the tire can support up to 1,653 pounds per tire (the load index is a code, not a direct measurement). The H means the tire is rated for sustained speeds up to 130 mph. Common speed ratings include S (112 mph), T (118 mph), H (130 mph), V (149 mph), W (168 mph), and Y (186 mph). Always replace tires with the same or higher load index and speed rating as the originals.
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