Tire Size Calculator: Find Your Exact Fit

This tire size calculator works out the overall diameter, sidewall height, circumference, and revolutions per mile (or km) for any tire size you enter. You input the standard tire size numbers (e.g., 225/45R17) and get the physical dimensions that matter for fitment and speedometer accuracy. The result appears instantly, along with the formulas used so you can verify the math yourself.

Formula

The overall diameter (D) is calculated as:

D = (2 × (W × R / 100) / 25.4) + r

Where:

• W = tire width in millimeters (e.g., 225)
• R = aspect ratio as a percentage (e.g., 45 means sidewall height is 45% of width)
• r = rim diameter in inches (e.g., 17)

The sidewall height in millimeters is simply W × R / 100. The circumference is π × D. Revolutions per mile uses 63,360 inches per mile divided by circumference.

Worked Example

Using the default values (225/45R17):

1. Sidewall height = 225 × 45 / 100 = 101.25 mm
2. Convert to inches: 101.25 / 25.4 = 3.9862 inches
3. Overall diameter = (2 × 3.9862) + 17 = 24.9724 inches
4. Circumference = π × 24.9724 = 78.452 inches
5. Revs per mile = 63,360 / 78.452 = 807.6 → 808 revs/mile

The calculator shows 24.97 inches diameter, 101 mm sidewall, 78.45 inches circumference, and 808 revs/mile. All match.

How to Read a Tire Size

Tire sizes follow a standard format: P225/45R17 (the leading P is optional and means passenger vehicle). The first number is width in millimeters. The second is the aspect ratio: the sidewall height as a percentage of width. The R means radial construction. The last number is the rim diameter in inches.

For example, a 265/70R17 has a much taller sidewall (70% of 265 = 185.5 mm) than a 225/45R17 (101.25 mm). This is why the same rim size can look very different.

Why Tire Size Matters

Changing tire diameter affects several things:

• Speedometer accuracy: A larger diameter tire travels farther per revolution, so your speedometer reads lower than actual speed.
• Ground clearance: Bigger tires raise the vehicle, useful for off-road but can affect handling.
• Gearing: Larger tires effectively change final drive ratio, reducing acceleration but increasing top speed.
• Clearance: A tire that is too wide or too tall may rub against fenders, suspension, or brakes.

Always check your owner’s manual or the sticker on the driver’s door jamb for the original tire size. If you deviate, keep the overall diameter within 3% of the original to avoid serious issues.

Common Mistakes and Misconceptions

One frequent error is mixing units. Tire width is in millimeters, rim diameter in inches. The calculator handles the conversion, but if you do it manually, remember 1 inch = 25.4 mm.

Another misconception: the aspect ratio is not a fixed sidewall height. A 45-series tire on a 225 width has a 101 mm sidewall, but on a 255 width it’s 114.75 mm. The percentage matters, not the number alone.

Also, the “R” in 225/45R17 is often mistaken for “rim” but it means radial construction. Most modern tires are radial.

How to Choose a Replacement Tire

To find a comparable tire, you can use the calculator to compare diameters. For example, if you have 225/45R17 (24.97 inches) and want wider tires, try 235/45R17:

• Sidewall = 235 × 0.45 = 105.75 mm
• Diameter = (2 × 105.75 / 25.4) + 17 = 25.33 inches
• Difference = 0.36 inches (1.4%), which is acceptable.

If the difference exceeds 3%, you may need to adjust tire pressure, speedometer calibration, or even change rim size.

Limits of the Calculator

This calculator gives theoretical dimensions based on the numbers printed on the tire. Actual dimensions can vary slightly due to manufacturing tolerances, tread wear, and inflation pressure. A new tire vs. a worn tire can differ by several millimeters in diameter. Also, the tire’s load and speed ratings are not considered here—those are separate factors.

For exact fitment, always test-fit or consult a tire professional. The calculator is a planning tool, not a guarantee.

Frequently Asked Questions

What does the 3% rule mean?

Most manufacturers recommend keeping the overall diameter within 3% of the original. This minimizes speedometer error and avoids issues with ABS and traction control. Use the calculator to compare diameters and compute the percentage difference.

Can I use a tire with a different aspect ratio?

Yes, but you must adjust the width or rim size to keep the diameter similar. For example, going from 225/45R17 to 225/40R17 reduces diameter by about 0.9 inches (3.6%), which is too much. A 235/40R17 would be closer.

How do I know if a tire fits my car?

Besides diameter, check the wheel width range (printed in the tire spec) and the load index. The calculator only handles dimensions; fitment also depends on offset, brake clearance, and suspension geometry. When in doubt, ask a tire shop.

Why does my speedometer read high after installing bigger tires?

Bigger tires cover more distance per revolution, so the speedometer (which counts wheel rotations) thinks you’re going slower than you are. To correct it, you may need a speedometer recalibration or a different gear.

What is the difference between tire size and wheel size?

Tire size includes the width, aspect ratio, and rim diameter (e.g., 225/45R17). Wheel size is just the rim diameter (17 inches). The tire is the rubber part; the wheel is the metal rim.

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in width | Tire width | mm | 225 | 0
in ratio | Aspect ratio | % | 45 | 0
in rim | Rim diameter | inches | 17 | 0
out diameter | Overall diameter | inches | ((width * ratio / 100) * 2 / 25.4) + rim | 2
out sidewall | Sidewall height | mm | width * ratio / 100 | 0
out circumference | Circumference | inches | pi * (((width * ratio / 100) * 2 / 25.4) + rim) | 2
out revs | Revs per mile | revs/mile | 63360 / (pi * (((width * ratio / 100) * 2 / 25.4) + rim)) | 0