How to Read Tire Sizes
Tire sizes follow a standardized format: width/aspect_ratioR wheel_diameter. For example, 225/45R17 means the tire is 225mm wide, has an aspect ratio of 45% (sidewall height is 45% of width), is radial construction, and fits a 17-inch wheel. Understanding these numbers helps you make informed decisions when replacing or upgrading tires.
The width affects contact patch and handling, while the aspect ratio influences ride comfort and sidewall stiffness. Lower aspect ratios (performance tires) provide better handling but harsher rides, while higher ratios offer more comfort but less responsive handling. The wheel diameter must match your existing wheels unless you're also changing wheels.
Speedometer Impact
Changing tire size affects your speedometer accuracy because it measures wheel rotations to calculate speed. Larger tires cover more distance per rotation, causing your speedometer to read slower than your actual speed. Conversely, smaller tires make it read faster. This calculator shows the exact percentage difference.
A 3% difference is generally considered acceptable, but larger differences can lead to speeding tickets or inaccurate mileage tracking. Modern vehicles can sometimes be recalibrated by dealers, but awareness of the difference helps you adjust your driving accordingly. GPS speed measurement apps can help verify your actual speed.
Common Use Cases
Plus sizing: Moving to larger wheels with lower aspect ratio tires (like 225/45R17 to 235/40R18) maintains similar overall diameter while improving handling. This calculator ensures you stay within acceptable size limits.
Winter tires: Many drivers use narrower winter tires for better snow performance. Compare your summer and winter tire sizes to understand the impact on speedometer and clearance.
Replacement options: When your exact tire size isn't available, find the closest alternative that maintains similar diameter. This helps preserve speedometer accuracy and vehicle performance characteristics.
Frequently Asked Questions
How do I read tire size numbers?
Tire size is written as width/aspect_ratioR wheel_diameter. For example, 225/45R17 means 225mm width, 45% aspect ratio, radial construction, 17-inch wheel.
Will changing tire size affect my speedometer?
Yes. Larger tires make your speedometer read slower than actual speed, while smaller tires make it read faster. This calculator shows the exact percentage difference.
What is tire aspect ratio?
Aspect ratio is the height of the tire sidewall as a percentage of the width. A 45 aspect ratio means the sidewall height is 45% of the tire width.
Can I use different tire sizes on my car?
You can use different sizes if they maintain similar overall diameter (within 3%) and fit your wheel wells. Always check clearance and consult your vehicle manual.
How much difference is acceptable?
Generally, stay within 3% of the original tire diameter to avoid significant speedometer error and potential issues with vehicle systems.
Privacy and Accuracy
All tire calculations run entirely in your browser. No tire sizes, vehicle information, or calculations are sent to any server. Your data stays completely private on your device. The tool uses React state management for instant updates as you adjust values — no loading screens, no API calls, no accounts.
Whether you're comparing performance tires, planning winter tire setup, or just curious about size differences, the calculator renders instantly on desktop, tablet, and mobile. The responsive layout adapts to any screen size while maintaining full functionality including the comparison feature and detailed dimension breakdowns.