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About This Calculator
Tire size designations like 225/45R17 encode width, aspect ratio, and wheel diameter in a format that is not intuitive to most drivers. This calculator decodes these specifications into actual physical dimensions including overall diameter, sidewall height, circumference, and revolutions per mile. It also compares two tire sizes to show speedometer error, ground clearance change, and whether the alternate size is safe for your vehicle.
Quick Tips
- 1 The first number in tire size (e.g., 225) is the tread width in millimeters.
- 2 Plus-sizing by one inch changes handling but keep overall diameter within 3%.
- 3 The aspect ratio is the sidewall height as a percentage of tread width.
Example Calculation
Compare 225/45R17 (original) vs 235/40R18 (upgrade).
Original diameter: 25.97 in | New: 25.39 in | Difference: -2.21% | Speedometer reads 2.3% fast
Reading Tire Size Notation
A tire marked 225/45R17 has a width of 225mm, an aspect ratio of 45 (sidewall height is 45% of width), and fits a 17-inch wheel. The R indicates radial construction. Understanding this notation is essential for choosing replacement tires.
How Overall Diameter Is Calculated
The overall tire diameter equals the wheel diameter plus twice the sidewall height. Sidewall height is the tire width multiplied by the aspect ratio divided by 100, then converted from millimeters to inches (divide by 25.4).
Revolutions Per Mile
Revolutions per mile indicates how many times the tire rotates over one mile. It is calculated by dividing 63,360 (inches per mile) by the tire circumference in inches. This affects your speedometer accuracy and odometer readings.
Effects of Changing Tire Size
Changing tire size affects speedometer accuracy, ride quality, handling, and fuel economy. A larger diameter tire makes the speedometer read slower than actual speed. Keep the overall diameter within 3% of the original to avoid problems.
Frequently Asked Questions
Larger tires cover more ground per revolution, making the speedometer read lower than actual speed. For example, a 3% larger tire means your speedometer shows 60 mph when you are actually traveling 61.8 mph.
The aspect ratio is the sidewall height expressed as a percentage of the tire width. A 225/45 tire has a sidewall height of 225 x 0.45 = 101.25mm. Lower aspect ratios mean shorter sidewalls, which improve handling but reduce ride comfort.
You can change tire size within limits. Keep the overall diameter within 3% of the original to maintain speedometer accuracy and avoid clearance issues. Consult your vehicle manual or a tire professional for safe alternative sizes.
Plus-sizing increases wheel diameter while decreasing the aspect ratio to maintain overall diameter. Minus-sizing does the opposite. For example, going from 16-inch to 17-inch wheels with a lower profile tire keeps the same overall diameter.