Tire Size Calculator

Understanding Tire Size Numbers

A tire size like 225/45R17 contains three key measurements. The first number (225) is the tire width in millimeters from sidewall to sidewall. The second number (45) is the aspect ratio: the sidewall height as a percentage of the width, so 45% of 225mm equals 101.25mm. The letter R indicates radial construction. The last number (17) is the wheel diameter in inches. This calculator converts between tire sizes, compares dimensions, and shows the real-world impact of changing tire sizes.

How Tire Size Affects Your Vehicle

Changing tire size affects speedometer accuracy, ride comfort, handling, fuel economy, and ground clearance. A taller tire increases ground clearance and makes the speedometer read slower than actual speed. A wider tire improves grip but may increase road noise and fuel consumption. Most manufacturers recommend staying within 3% of the original tire overall diameter to keep speedometer error, ABS function, and transmission behavior within acceptable limits.

Comparing Tire Sizes

When upgrading wheels from 16-inch to 18-inch, you need a lower profile tire to maintain the same overall diameter. For example, switching from 205/55R16 to 225/40R18 keeps the diameter similar while giving a wider contact patch and sportier look. The calculator shows the exact difference in diameter, circumference, sidewall height, and speedometer variation so you can make an informed decision before purchasing.

Speedometer Correction

Your speedometer is calibrated for the original tire size. Larger tires cause the speedometer to read low: when it shows 60 mph, you might actually be doing 63 mph. Smaller tires cause the opposite. This matters for speed limit compliance, odometer accuracy, and even resale value if the odometer reading does not match actual distance driven. Use this calculator to find the exact speedometer error percentage for any tire size change.

Frequently asked questions