How the tire size calculator works
Enter your current tire size and a new size using the three numbers from the sidewall, width in millimeters, aspect ratio as a percentage, and wheel diameter in inches, and the tool compares the overall diameters. It shows how much taller or shorter the new tire is, the percent difference, and how the change affects your speedometer reading.
Reading a tire size
A size like 225/45R17 means 225 mm wide, a sidewall height that is 45 percent of the width, on a 17 inch wheel. Overall diameter is the wheel diameter plus twice the sidewall height. Changing any of the three numbers changes the rolling diameter, which is what matters for fit and for accuracy of your speedometer and odometer.
Speedometer and odometer error
Your speedometer is calibrated for the original tire diameter. A larger tire travels farther per revolution, so the car actually goes faster than the dial shows and logs fewer miles than it travels; a smaller tire does the opposite. The tool shows your true speed when the speedometer reads 60 so you can judge the impact.
Staying within a safe range
A common guideline is to keep a replacement tire within about 3 percent of the original diameter so that speedometer error, clearance, and on some vehicles the anti-lock and stability systems stay within tolerance. Larger changes may rub, alter gearing, or trigger warnings. Always confirm fitment with a tire professional before changing sizes.
Frequently asked questions
Will bigger tires change my speedometer? Yes, a larger diameter makes your true speed higher than the speedometer shows.
How much tire size change is safe? A common rule is within about 3 percent of the original overall diameter.
What do the tire numbers mean? Width in mm, aspect ratio as a percent of width, and wheel diameter in inches.
Related calculators: Fuel Cost, MPG, Brake Distance.