Tire Size Calculator
Convert metric tire sizes to inches, compare two tires, and see speedometer impact.
Tire 1 Original
Tire 2 Comparison
Side-by-Side Comparison
How to Read a Tire Size
A tire size like 225/45R17 can be broken down into three parts: 225 is the tire width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), and 17 is the wheel diameter in inches. The “R” stands for radial construction. Understanding these numbers helps you pick the right tire for your vehicle.
Tire Size Conversion Formula
To convert a metric tire size to inches, the calculator uses these equations:
Overall Diameter (inches) = (2 × Sidewall Height) + Wheel Diameter
Circumference (inches) = Ï€ × Diameter
Revolutions per Mile = 63,360 / Circumference
For example, a 225/45R17 tire has a sidewall of (225×0.45)/25.4 ≈ 3.99 inches, giving an overall diameter of about 24.98 inches. The tool does all the math instantly.
Why Compare Tire Sizes?
Switching to a different tire size can affect your speedometer accuracy, ground clearance, and gearing. A larger diameter tire will make your speedometer read slower than actual speed, while a smaller diameter does the opposite. This calculator shows the percentage difference and predicts the speedometer change at 60 mph, helping you make an informed decision.
Frequently Asked Questions
- Can I use this tool for truck tires? Yes, it works for any metric tire size (including LT and P-metric). Just enter width, aspect ratio, and wheel size.
- What does “revolutions per mile” mean? It’s the number of times the tire rotates in one mile. This value is used for odometer and speedometer calibration.
- How accurate is the speedometer difference? It’s a theoretical estimate based purely on tire diameter. Actual speedometer error may vary due to vehicle tolerances.
Bookmark this tire size calculator and use it whenever you’re shopping for new wheels or tires!
