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Tire Size Calculator

This tool calculates and compares the geometric dimensions of automotive tires based on their ISO metric codes. By inputting the specifications of your current tire and a proposed replacement, you can quantify changes in overall diameter, sidewall height, and circumference.

Modifying tire sizes affects vehicle dynamics, including speedometer calibration, ground clearance, and effective gear ratio. This calculator applies standard engineering formulas to determine the percentage difference between two tire sizes, helping to identify fitments that remain within manufacturer safety tolerances.

Updated Dec 1, 2025

Compare two standard metric tire sizes (e.g., 205/55R16).

Current Tire Size

New Tire Size

Results

Stock Diameter

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New Diameter

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Difference

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True Speed @ 65mph

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Methodology

The calculator derives tire dimensions using the ISO 4000-1 standard naming convention (e.g., 225/45R17). The section width is converted from millimeters to inches. The sidewall height is calculated as the product of the section width and the aspect ratio (percentage).

Total tire diameter is the sum of the rim diameter and twice the calculated sidewall height. Circumference is the product of the diameter and Pi (π). Revolutions per mile are derived by dividing 63,360 (inches in a mile) by the tire circumference.

Speedometer error is calculated by comparing the rotational distance of the new tire against the original equipment (stock) tire. If the new tire has a larger circumference, it travels further per revolution, causing the speedometer to read lower than the vehicle's actual ground speed.

Expert Q&A

What is the 3% rule for tire sizing?

Automotive engineers and safety experts generally recommend keeping the total diameter of replacement tires within 3% of the original equipment (OEM) specification. Exceeding this limit can adversely affect anti-lock braking systems (ABS), traction control, and transmission shift points.

How does tire size affect speedometer accuracy?

Speedometers are calibrated to the circumference of the factory-installed tires. A larger tire rotates fewer times to cover the same distance, causing the speedometer to display a speed lower than your actual velocity. Conversely, a smaller tire will cause the speedometer to read higher than your true speed.

What do the numbers on a tire sidewall mean?

The first number (e.g., 205) is the width of the tire in millimeters. The second number (e.g., 55) is the aspect ratio, indicating the sidewall height is 55% of the width. The third number (e.g., 16) is the diameter of the wheel (rim) in inches.

Sources & citations