Tire Size Calculator – Compare Sizes & Speedometer Difference
Original Tire (Tire 1)
New Tire (Tire 2)
| Metric | Tire 1 | Tire 2 | Difference |
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How to Read and Compare Tire Sizes
Tire sizes follow a standardized format like 225/45R17. The first number (225) is the section width in millimeters, which is the widest point of the tire from sidewall to sidewall. The second number (45) is the aspect ratio, representing the sidewall height as a percentage of the width. So on a 225/45 tire, the sidewall is 45% of 225mm, or 101.25mm. The letter R indicates radial construction, and the final number (17) is the wheel diameter in inches.
The overall diameter of the tire is calculated by adding the rim diameter to twice the sidewall height. This determines the circumference, which directly affects how far the car travels per wheel revolution. When you change tire sizes, the difference in circumference causes your speedometer and odometer to read incorrectly. A larger tire covers more ground per revolution, making the speedometer read lower than actual speed, while a smaller tire makes it read higher.
This calculator compares two tire sizes and shows the differences in every key dimension including diameter, width, sidewall height, circumference, and revolutions per mile. It also calculates the exact speedometer error so you know what your actual speed will be at any displayed speed. Most experts recommend keeping tire diameter changes within 3% of the original equipment size to avoid issues with ABS, traction control, and transmission calibration.
Frequently Asked Questions
How do I read a tire size like 225/45R17?
225 is the width in mm, 45 is the aspect ratio (sidewall height as % of width), R means radial, and 17 is the wheel diameter in inches. So the sidewall is 225 x 0.45 = 101.25mm.
How does changing tire size affect my speedometer?
Larger tires make your actual speed higher than displayed; smaller tires make it read high. A 3% diameter increase means your true speed is about 3% faster than shown.
What is the maximum tire size change I should make?
Stay within 3% of the original diameter. Larger changes can affect ABS, traction control, ride quality, and cause clearance issues. Always consult your vehicle's manual.
What are revolutions per mile?
Revolutions per mile is how many times the tire rotates in one mile. Smaller tires have more revolutions, larger tires have fewer. This affects speedometer accuracy, odometer readings, and fuel economy.