Tire Size Calculator

How the Tire Size Calculator Works

This calculator compares two tire sizes and shows how the change affects key measurements. It uses standard tire size notation (e.g., 225/45R17) where the first number is the section width in millimeters, the second is the aspect ratio as a percentage of width, and the third is the wheel diameter in inches.

The calculator computes the sidewall height, overall diameter, circumference, and revolutions per mile for each tire. It then calculates the difference in diameter and the resulting speedometer error when switching from one size to another.

How to Use the Calculator

1. Enter the current tire size in the first set of fields (width, aspect ratio, wheel diameter).
2. Enter the new tire size in the second set of fields.
3. The results will automatically display the differences in diameter, circumference, and speedometer reading.

Understanding the Results

Diameter and Circumference

The overall diameter is the total height of the tire from the ground to the top of the tread. Circumference is the distance the tire travels in one revolution. A larger diameter tire will have a larger circumference, meaning it covers more ground per rotation.

Speedometer Error

When you change tire diameter, your speedometer reading changes because it measures wheel rotations, not actual ground speed. A larger tire covers more distance per rotation, so your speedometer will read slower than your actual speed. A smaller tire does the opposite. The calculator shows the difference as a percentage and as a speed reading at a given reference speed (typically 60 or 65 mph).

Revolutions Per Mile

This number tells you how many times the tire rotates to cover one mile. It is useful for calibrating speedometers and odometers after a tire size change.

Common Mistakes When Changing Tire Size

• Ignoring clearance issues: A larger diameter tire may rub against the fender, suspension components, or the inside of the wheel well, especially at full steering lock or under compression.
• Overlooking width changes: A wider tire may contact suspension parts or the inner fender liner. It also affects steering feel and may require different wheel offset.
• Assuming speedometer error is negligible: Even a small diameter change (3-5%) can result in a noticeable speedometer discrepancy that may lead to speeding tickets or inaccurate mileage tracking.
• Not considering load rating and speed rating: The new tire must meet or exceed the vehicle's original load and speed ratings for safety.

Limitations of This Calculator

This calculator provides theoretical measurements based on tire size specifications. Actual tire dimensions can vary slightly between manufacturers and tire models due to tread design, construction, and inflation pressure. The speedometer error calculation assumes the original tire size was the factory specification and that the vehicle's speedometer was accurate with that size. Real-world factors like tire wear, inflation pressure, and vehicle-specific calibration may cause minor deviations.

Practical Use Cases

• Upgrading to larger wheels: Determine how a larger wheel diameter with a lower-profile tire affects overall diameter and speedometer accuracy.
• Switching to winter tires: Compare a narrower winter tire size to maintain similar diameter while improving snow traction.
• Fitting off-road tires: Check if a taller all-terrain tire will fit within the wheel well and how much it will throw off the speedometer.
• Replacing unavailable sizes: Find an alternative tire size that closely matches the original diameter when the exact size is discontinued or out of stock.