Tire Size Calculator
Calculate tire dimensions from standard tire size notation. Find overall diameter, circumference, sidewall height, and revolutions per mile. Compare two tire sizes to see speedometer error.
Format: Width(mm) / Aspect Ratio(%) R Rim Diameter(inches)
How to Read Tire Sizes
A tire size like 225/45R17 contains three key measurements. The first number (225) is the section width in millimeters — the width of the tire from sidewall to sidewall. The second number (45) is the aspect ratio — the sidewall height as a percentage of the width. The letter R indicates radial construction. The last number (17) is the rim diameter in inches. Together, these determine the tire's overall dimensions.
Sidewall Height = Width x (Aspect Ratio / 100)
Overall Diameter = (Rim x 25.4) + (2 x Sidewall Height)
Circumference = Overall Diameter x pi
Revolutions/Mile = 1,609,344 / Circumference
Tire Size Anatomy Diagram
Plus Sizing Explained
Plus sizing means increasing rim diameter while decreasing sidewall height to maintain the same overall tire diameter. For example, moving from 225/45R17 to 235/40R18 keeps the overall diameter similar while providing a wider contact patch and more responsive handling. The trade-off is a firmer ride due to less sidewall flex. When plus sizing, aim to keep the overall diameter within 3% of the original to avoid speedometer errors and clearance issues.
Common Tire Sizes Reference
| Tire Size | Diameter (in) | Width (in) | Sidewall (in) | Rev/Mile |
|---|---|---|---|---|
| 195/65R15 | 25 | 7.7 | 5 | 832 |
| 205/55R16 | 24.9 | 8.1 | 4.4 | 833 |
| 215/60R16 | 26.2 | 8.5 | 5.1 | 791 |
| 225/45R17 | 25 | 8.9 | 4 | 831 |
| 225/50R17 | 25.9 | 8.9 | 4.4 | 802 |
| 235/40R18 | 25.4 | 9.3 | 3.7 | 817 |
| 245/45R18 | 26.7 | 9.6 | 4.3 | 778 |
| 255/35R19 | 26 | 10 | 3.5 | 798 |
| 265/40R20 | 28.3 | 10.4 | 4.2 | 734 |
| 275/55R20 | 31.9 | 10.8 | 6 | 651 |
Speedometer Correction
Your speedometer is calibrated for the original tire size. If you change to a tire with a different overall diameter, the speedometer will be inaccurate. A larger tire means you travel farther per revolution, so your actual speed is higher than displayed. A smaller tire means you travel less per revolution, so actual speed is lower. Most jurisdictions allow up to 3% speedometer error. Beyond that, you may need recalibration or a speedometer correction device.
Frequently Asked Questions
What does the R in tire size mean?
The R stands for Radial construction, which is the standard for modern passenger tires. Radial tires have cord plies arranged at 90 degrees to the direction of travel, providing better grip and longer life than older bias-ply designs.
Can I use a different tire size than recommended?
You can use different sizes within limits. Keep the overall diameter within 3% of the original to avoid speedometer errors, ABS issues, and clearance problems. The width should not exceed what your rim can safely support (typically +/- 10mm from the rim's recommended width).
What is a low-profile tire?
A low-profile tire has an aspect ratio of 50 or less (e.g., 225/40R18). The shorter sidewall provides sharper handling and a sportier look but results in a firmer ride and increased vulnerability to pothole damage.
How does tire width affect performance?
Wider tires provide a larger contact patch for better grip in dry conditions and improved cornering. However, they increase rolling resistance (reducing fuel economy), are more prone to hydroplaning, and may not fit in all wheel wells.
What are revolutions per mile used for?
Revolutions per mile indicates how many times the tire rotates to cover one mile. This affects speedometer accuracy, odometer readings, and gear ratio calculations. Fewer revolutions per mile means a larger tire diameter.
How do I find my current tire size?
Your tire size is printed on the sidewall of your current tires. You can also find the recommended size on a sticker inside the driver's door jamb, in your owner's manual, or on the fuel filler door.