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Tire Size Calculator

Tire sizes follow a standardized format like 225/50R17, where 225 is the section width in millimeters, 50 is the aspect ratio (sidewall height as a percentage of width), and 17 is the rim diameter in inches. Overall tire diameter is calculated as (2 x sidewall / 25.4) + rim diameter. Enter a tire size below to parse its dimensions, or enter two sizes to compare diameter difference and speedometer impact.

Quick Answer

A 225/50R17 tire has a sidewall height of 112.50 mm, an overall diameter of 25.86 inches, a circumference of 81.22 inches, and makes approximately 780 revolutions per mile.

Parse Tire Size

Format: 225/50R17 or P225/50R17

Compare Tire Sizes

Common Examples

Input Result
225/50R17 25.86" diameter, 81.22" circumference, 780 rev/mile
265/70R17 31.61" diameter, 99.29" circumference, 638 rev/mile
205/55R16 24.88" diameter, 78.13" circumference, 811 rev/mile
225/50R17 vs 235/55R17 Diameter diff: +1.44", speedometer reads 5.58% slow

How It Works

This calculator uses standard tire geometry formulas to convert the metric/inch tire size code into usable dimensions.

Tire Size Format: [Width]/[Aspect Ratio]R[Rim Diameter]

For example, 225/50R17 means:

  • 225 = section width in millimeters (the tire’s width from sidewall to sidewall)
  • 50 = aspect ratio (sidewall height is 50% of the section width)
  • R = radial construction
  • 17 = rim diameter in inches

Dimension Formulas

Sidewall Height (mm) = Width x Aspect Ratio / 100

Overall Diameter (inches) = (Sidewall x 2 / 25.4) + Rim Diameter

The sidewall is multiplied by 2 because the tire has sidewall on both the top and bottom. Dividing by 25.4 converts millimeters to inches. Adding the rim diameter gives the total tire diameter.

Circumference (inches) = PI x Diameter

Revolutions per Mile = 63,360 / Circumference

There are 63,360 inches in a mile (5,280 feet x 12 inches).

Speedometer Difference

When changing tire sizes, the speedometer reading is affected because it is calibrated for the original tire’s revolutions per mile. The speedometer difference is:

Speedometer Difference (%) = ((Original Rev/Mile / New Rev/Mile) - 1) x 100

A positive value means the speedometer reads slower than actual speed (the new tire is larger). A negative value means it reads faster (the new tire is smaller). A difference within 0.5 inches of diameter is generally considered an acceptable substitution.

Worked Example

For a 225/50R17 tire: Sidewall = 225 x 50 / 100 = 112.5 mm. Diameter = (112.5 x 2 / 25.4) + 17 = (225 / 25.4) + 17 = 8.858 + 17 = 25.86 inches. Circumference = 3.14159 x 25.86 = 81.22 inches. Revolutions per mile = 63,360 / 81.22 = 780.

Comparing 225/50R17 (25.86” diameter) to 235/55R17 (27.30” diameter): Diameter difference = +1.44 inches. The new tire makes fewer revolutions per mile, so the speedometer reads approximately 5.58% slower than actual speed at any given velocity.

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Frequently Asked Questions

What do the numbers in a tire size mean?
A tire size like 225/50R17 has three components: 225 is the section width in millimeters, 50 is the aspect ratio (sidewall height as a percentage of width), and 17 is the rim diameter in inches. The 'R' indicates radial construction, which is the standard for modern passenger tires. An optional 'P' prefix indicates a passenger tire.
How does changing tire size affect the speedometer?
The speedometer is calibrated for the original tire diameter. A larger tire covers more ground per revolution, making the speedometer read slower than actual speed. A smaller tire makes the speedometer read faster. For every 1 inch change in diameter, the speedometer error is roughly 3% to 4%, depending on the original tire size.
What is an acceptable tire size difference?
A general guideline is that the new tire diameter should be within 3% of the original (roughly 0.5 inches for most passenger tires). Larger differences can affect speedometer accuracy, ABS function, traction control calibration, and may cause rubbing on wheel wells or suspension components.
What is aspect ratio?
The aspect ratio is the height of the tire's sidewall expressed as a percentage of its section width. A 225/50 tire has a sidewall that is 50% of 225 mm, which equals 112.5 mm. A lower aspect ratio means a shorter, stiffer sidewall (sportier handling). A higher aspect ratio means a taller sidewall (more cushion, better for rough roads).
Why do tire sizes mix metric and imperial units?
The tire sizing convention was established internationally and uses millimeters for width (metric) and inches for rim diameter (imperial). This mixed system became the industry standard and persists today. The aspect ratio is a pure percentage with no unit. Calculators like this one handle the unit conversion automatically.