Actual Tire Size Calculator: Compare Tire Dimensions

An actual tire size calculator estimates a tire’s outside diameter, circumference, revolutions per mile, and change from another tire size using the numbers printed on its sidewall. Enter the tire width, aspect ratio, and wheel diameter for each size to see the calculated dimensions and compare them; the result is an estimate, not a substitute for checking vehicle clearance or manufacturer fitment data.

Formula

A tire code such as 225/60R17 contains three measurements used by this calculator: nominal section width, aspect ratio, and wheel diameter. The letter R indicates radial construction; it is not a measurement in the diameter formula.

• Sidewall height = width × (aspect ratio ÷ 100). Width and sidewall height are in millimetres.
• Outside diameter = wheel diameter + 2 × (sidewall height ÷ 25.4). The result is in inches; 25.4 millimetres equal one inch.
• Circumference = π × outside diameter, in inches.
• Diameter difference = 100 × (new diameter − comparison diameter) ÷ comparison diameter, as a percentage.
• Estimated revolutions per mile = 63,360 ÷ circumference in inches. There are 63,360 inches in a statute mile.

In the calculator, w is the first tire’s width in millimetres, a is its aspect ratio as a percentage, and r is its wheel diameter in inches. Variables with a 2 refer to the comparison tire. The diameter is the main result; the other outputs describe related dimensions and the relative change.

Worked Example

Using the defaults, the first tire is 225/60R17 and the comparison tire is 205/55R16.

1. First tire sidewall height: 225 × 60 ÷ 100 = 135 mm.
2. Convert the sidewall height to inches and add both sidewalls to the wheel diameter: 17 + 2 × (135 ÷ 25.4) = 27.6299 inches, or 27.63 inches.
3. Circumference: π × 27.6299 = 86.798 inches, or 86.80 inches.
4. Comparison tire diameter: 16 + 2 × (205 × 55 ÷ 100 ÷ 25.4) = 24.8780 inches.
5. Diameter difference: 100 × (27.6299 − 24.8780) ÷ 24.8780 = 11.06%.
6. Estimated revolutions per mile: 63,360 ÷ 86.798 = 729.5, rounded to 730 rev/mi.

The calculator rounds displayed outputs, but it uses the calculated values internally. That matters when checking the percentage or revolutions per mile: calculating from already-rounded display figures can produce a slightly different last digit.

How to Read a Tire Size

For 225/60R17, 225 is the nominal width in millimetres, 60 means the sidewall height is 60% of that width, and 17 is the wheel diameter in inches. The R identifies radial construction. A wider tire does not necessarily have a taller sidewall: the sidewall depends on width multiplied by the aspect ratio.

The word “actual” can be misleading. The calculation gives a nominal geometric estimate from standardized size markings. A mounted tire’s measured dimensions can differ among tire models and can be affected by the approved measuring rim, inflation, load, tread wear, and construction. For exact fitment, use the tire maker’s specifications for the particular model and size.

When to Use the Comparison

Compare sizes when considering a replacement size, checking how a proposed change affects overall diameter, or estimating how tire circumference relates to distance and speed readings. The percentage is measured against the comparison size entered in the second set of fields. A positive result means the first tire’s calculated diameter is larger; a negative result means it is smaller.

A diameter change also changes the estimated distance travelled per wheel revolution. If a vehicle’s speedometer or odometer is calibrated for the comparison tire, a larger tire generally travels farther per revolution than that calibration assumes. The calculator does not predict the exact indicated speed or odometer error: calibration, tire rolling circumference, tread condition, and vehicle systems also matter.

Common Errors and Limits

• Entering the wrong units: width must be in millimetres and wheel diameter in inches. Do not enter a wheel diameter in millimetres or a width in inches.
• Using aspect ratio as a decimal: enter 60 for a 60-series tire, not 0.60 or 60 mm.
• Comparing unlike measurements: the calculator compares nominal dimensions, not tread width, rim width, loaded radius, or measured rolling circumference.
• Assuming a similar diameter guarantees fit: width, suspension travel, steering angle, wheel offset, brake clearance, load capacity, and vehicle-specific requirements can all affect fit.
• Treating revolutions per mile as exact: it is derived from the estimated circumference. A tire’s published revolutions-per-mile value may use a measured rolling circumference and can differ.

Use the vehicle placard or owner information and tire manufacturer data to verify an appropriate replacement. A mathematical comparison cannot establish safe clearance, suitable load or speed capability, or compatibility with the wheel.

Frequently Asked Questions

How do I calculate tire diameter from tire size?
Multiply tire width by the aspect ratio divided by 100 to find one sidewall’s height in millimetres. Convert that height to inches, double it, and add the wheel diameter.

What does 225/60R17 mean?
It describes a nominal 225 mm section width, a sidewall height equal to 60% of that width, radial construction, and a 17-inch wheel diameter.

Is calculated tire diameter the same as measured diameter?
No. The formula estimates nominal outside diameter. The measured size can vary by tire model, rim, inflation, load, and wear; consult the tire maker for model-specific specifications.

How much tire diameter difference is acceptable?
There is no universal percentage that guarantees a replacement will fit or function correctly. Check the vehicle maker’s approved sizes and verify clearance, load requirements, and compatibility rather than relying on diameter alone.

Does a larger tire make the speedometer read low?
If the vehicle remains calibrated for a smaller tire, a larger rolling circumference generally means the vehicle travels farther per wheel revolution, so the indicated speed may be lower than actual. The precise difference depends on the tire’s rolling circumference and vehicle calibration.

Why is my tire’s published revolutions-per-mile figure different?
This calculator estimates revolutions from nominal diameter and circumference. Published figures can be based on measured rolling circumference under specified conditions, so they need not match the geometric estimate.

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in w | Tire width | mm | 225 | 0
in a | Aspect ratio | % | 60 | 0
in r | Wheel diameter | in | 17 | 0
in w2 | Comparison width | mm | 205 | 0
in a2 | Comparison aspect ratio | % | 55 | 0
in r2 | Comparison wheel diameter | in | 16 | 0
out diameter | Actual tire diameter | in | r + 2 * (w * a / 100 / 25.4) | 2
out circumference | Tire circumference | in | pi * diameter | 2
out difference | Diameter difference | % | 100 * (diameter - (r2 + 2 * (w2 * a2 / 100 / 25.4))) / (r2 + 2 * (w2 * a2 / 100 / 25.4)) | 2
out revs | Estimated revolutions per mile | rev/mi | 63360 / circumference | 0