Tire Size Comparison Calculator

How to Use the Tire Size Comparison Calculator

Our tire size comparison calculator helps you understand exactly how two tire sizes differ. Enter the width in millimeters, aspect ratio as a percentage, and rim diameter in inches for both your current tires and the tires you are considering. The calculator instantly shows you the overall diameter, sidewall height, circumference, revolutions per mile, and critically, the speedometer difference that a tire change would cause.

This tool is invaluable whether you are upgrading wheel sizes, switching between summer and winter tires, or exploring off-road tire options. Understanding the exact dimensional differences prevents clearance issues, speedometer errors, and potential drivetrain problems that come with mismatched tire sizes.

Understanding Tire Size Numbers

Tire sizes follow a standardized format such as 225/45R17. The first number (225) is the tire's section width in millimeters measured from sidewall to sidewall. The second number (45) is the aspect ratio, meaning the sidewall height is 45% of the width. The letter R indicates radial construction, and 17 is the wheel diameter in inches. The overall tire diameter combines the rim diameter plus twice the sidewall height.

Plus-Sizing and Minus-Sizing

Plus-sizing means increasing the rim diameter while decreasing the sidewall height to maintain roughly the same overall tire diameter. For example, moving from a 225/45R17 to a 235/40R18 keeps the diameter close while giving you a wider tire on a larger wheel. This approach improves handling and aesthetics while preserving speedometer accuracy. Minus-sizing does the opposite, typically for winter tire setups.

Speedometer and Odometer Effects

When you change tire sizes, your speedometer and odometer accuracy change proportionally to the diameter difference. A tire that is 3% larger means your speedometer reads about 3% low, so when it shows 60 mph, you are actually traveling around 61.8 mph. This also affects your odometer, meaning it records fewer miles than you actually drive. Most states and manufacturers consider up to a 3% error acceptable.

Frequently Asked Questions

How do I read tire size numbers?

A tire size like 225/45R17 means: 225 is the tire width in millimeters, 45 is the aspect ratio (sidewall height is 45% of the width), R means radial construction, and 17 is the rim diameter in inches.

How does tire size affect speedometer accuracy?

If you switch to larger tires, your vehicle travels farther per revolution than the speedometer expects, making it read slower than your actual speed. A 3% diameter increase means your actual speed is about 3% higher than displayed.

Can I put different size tires on my car?

You can change tire sizes within certain limits. Generally, the overall diameter should stay within 3% of the original tire to avoid speedometer errors, clearance issues, and drivetrain stress.

What happens if I use bigger tires?

Larger tires increase ground clearance but affect speedometer accuracy, may rub on fenders, can reduce acceleration, and may trigger ABS or traction control issues.

What is tire revolutions per mile?

Revolutions per mile is how many times a tire completes a full rotation to travel one mile, calculated by dividing 63,360 inches by the tire circumference.