Understanding the Tire Size Calculator
A tire size calculator decodes the numbers on your tire’s sidewall and shows how changing to a different size affects overall diameter, speedometer accuracy, and clearance. It is essential when considering new tires that differ from the factory specification.
Reading the sidewall code
A code like 225/45R17 tells you the tire’s width in millimeters, the aspect ratio (sidewall height as a percentage of width), and the wheel diameter in inches. The calculator converts these into the tire’s actual overall diameter and circumference, which is what determines how far the car travels per wheel revolution.
Why size changes matter
Switching to a tire with a different overall diameter changes your effective gearing and throws off your speedometer and odometer readings. A larger diameter makes the speedometer read low; a smaller one reads high. Big changes can also cause rubbing against the body or suspension, so checking the numbers before buying avoids costly mistakes.
Tips & things to know
- •Keep overall diameter close to stock to preserve speedometer accuracy.
- •A bigger tire makes your speedometer read slower than you are going.
- •Check clearance to avoid rubbing on the body or suspension.