Speedometers are calibrated based on your vehicle's original tire size. When you install larger or smaller tires, the number of revolutions per mile changes. Larger tires cover more distance per revolution, causing your speedometer to show a slower speed than your actual speed. Smaller tires have the opposite effect.

Generally, staying within 3% of your original tire diameter is considered safe. Beyond this, you may experience rubbing, speedometer inaccuracies, transmission issues, or ABS problems. Always consult your vehicle's manual or a tire professional before making significant changes.

The numbers indicate tire specifications: 225 is the width in millimeters, 45 is the aspect ratio (sidewall height as percentage of width), R indicates radial construction, and 17 is the wheel diameter in inches. The letter after the size (e.g., 91V) indicates load index and speed rating.

Larger tires are heavier and create more rolling resistance, which typically reduces fuel economy. Wider tires increase aerodynamic drag. However, taller tires can lower engine RPM at highway speeds, potentially improving fuel efficiency in some cases. The net effect depends on your driving patterns and vehicle.