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Tire Size Calculator

Parse any tire size, see all dimensions, and compare two tire sizes side by side. Find diameter differences, speedometer error, and circumference changes.

Quick Answer

A tire size like 225/55R17 means: 225mm tread width, 55% aspect ratio (sidewall height is 55% of width), R = radial, 17-inch rim. To calculate overall diameter: sidewall = 225 × 0.55 = 123.75mm (4.87"), diameter = 17 + 2 × 4.87 = 26.74". A 3% diameter change causes about 3% speedometer error.

Original Tire Size

New Tire Size

Tire Dimensions

Original: 225/55R17

Width225 mm

Aspect Ratio55%

Rim Diameter17"

Sidewall Height4.87"

Overall Diameter26.74"

Circumference84.02"

Revs per Mile754

New: 235/45R18

Width235 mm

Aspect Ratio45%

Rim Diameter18"

Sidewall Height4.16"

Overall Diameter26.33"

Circumference82.71"

Revs per Mile766

Comparison

-0.42"

Diameter Diff

-1.6%

Diameter Change

+1.6%

Speedometer Error

-0.21"

Ride Height Change

About This Tool

The Tire Size Calculator decodes any standard metric tire size and computes all physical dimensions. It also compares two tire sizes to show differences in diameter, circumference, speedometer accuracy, and ride height. This information is critical when upgrading wheels, switching to a different tire brand, or fitting aftermarket wheels.

How to Read Tire Size Numbers

A tire marked 225/55R17 contains three key measurements. The first number (225) is the tread width in millimeters. The second number (55) is the aspect ratio, meaning the sidewall height is 55% of the tread width. The letter R indicates radial construction (virtually all modern tires). The last number (17) is the wheel rim diameter in inches. From these three numbers, you can calculate every other dimension.

Speedometer Error Explained

Your speedometer calculates speed based on wheel rotation. It assumes a specific tire circumference. When you install tires with a different diameter, the circumference changes. A larger tire covers more ground per revolution, so your speedometer reads lower than actual speed. A smaller tire reads higher than actual speed. The error percentage equals the diameter difference percentage. Most experts recommend staying within 3% of the original tire diameter.

Plus-Size Fitting

Plus-sizing means increasing the wheel diameter while decreasing the sidewall height to maintain approximately the same overall tire diameter. For example, moving from 225/55R17 to 235/45R18. This keeps the speedometer accurate and preserves ride height while giving a sportier appearance and potentially improved handling from the stiffer, shorter sidewall. However, shorter sidewalls transmit more road imperfections, resulting in a firmer ride.

Frequently Asked Questions

What do tire size numbers mean?

In a tire size like 225/55R17: 225 is the width in millimeters, 55 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 17 is the rim diameter in inches. These three numbers define the tire's physical dimensions. The sidewall height would be 225mm × 0.55 = 123.75mm.

How much speedometer error is acceptable?

Most automotive experts and tire shops recommend staying within 3% of the original tire diameter, which translates to about a 3% speedometer error. At 60 mph, that is about 1.8 mph difference. Going beyond 3% may affect your warranty, cause inaccurate odometer readings, and could result in speeding tickets when you believe you are at the limit.

Can I put bigger tires on my car?

Yes, within limits. You need to ensure the larger tire fits within the wheel well without rubbing on suspension components, fenders, or body panels, especially at full steering lock or when hitting bumps. Keep the overall diameter within 3% of original to maintain speedometer accuracy. Check your vehicle's manual for approved tire size ranges.

What is plus-size tire fitting?

Plus-sizing increases the wheel diameter (e.g., from 17" to 18") while using a lower-profile tire (smaller aspect ratio) to keep the same overall diameter. This maintains speedometer accuracy and ride height while offering a more aggressive look and often better cornering performance, at the cost of a slightly firmer ride.

How does tire size affect fuel economy?

Wider tires create more rolling resistance, which slightly reduces fuel economy. Larger-diameter tires can improve highway fuel economy by reducing engine RPM at speed, but the heavier weight may offset this. The difference is typically 1-3%. Proper inflation has a much larger impact on fuel economy than tire size within normal ranges.

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