Tire Size Calculator
Know the difference before you buy.
Tire Size Calculator
Diameter = rim × 25.4 + 2 × (width × aspect ÷ 100)
Calculated instantly
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What This Calculator Does
The Tire Size Calculator compares an original tire with a replacement and reports the change in overall diameter and circumference, plus the speedometer error the change introduces. It reads the standard metric size notation, so a 205/55 R16 becomes an actual diameter in millimetres rather than three numbers you have to interpret.
How It Works
The rim diameter is converted from inches to millimetres. The sidewall height is the width multiplied by the aspect ratio, and because the tire has a sidewall on both sides of the wheel, it is counted twice. Adding the two sidewalls to the rim gives the overall diameter, and multiplying by pi gives the circumference. The percentage difference in diameter is also the speedometer error.
Example
A 205/55 R16 has a 16-inch rim (406.4 mm) and sidewalls of 205 × 0.55 = 112.75 mm, so its diameter is 631.9 mm. A 215/60 R17 comes to 689.8 mm, which is 9.2% larger. At an indicated 100 km/h the car is actually doing about 109 km/h, and the gearing is effectively taller.
Formula
sidewall = width × aspect ÷ 100diameter = rim × 25.4 + 2 × sidewallcircumference = π × diametererror% = (new diameter − old diameter) ÷ old diameter × 100Reading the Results
Diameter change
How much taller or shorter the new tire is. A positive number raises the car and reduces the effective gearing; a negative number does the opposite.
Speedometer error at 100 km/h
The percentage by which the speedometer under- or over-reads. A larger tire turns fewer times per kilometre, so the dashboard reads lower than the true speed.
Sources and References
Tire and Rim Association — Yearbook
Tire size designations and dimensions
The US standards body that defines the metric tire size notation used here.
ISO 4000-1 — Passenger car tyres and rims
Tyre designation and dimensions
The international standard for the size codes and measuring rim definitions.
UN Regulation No. 39 — Speedometer equipment
Speedometer accuracy requirements
Explains why a diameter change produces a legal as well as a practical speed-reading error.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
What do the numbers on a tire mean?
In 205/55 R16, 205 is the section width in millimetres, 55 is the sidewall height as a percentage of that width, R means radial construction, and 16 is the rim diameter in inches. The overall diameter is not written on the sidewall, which is why it has to be calculated.
How much size difference is acceptable?
Most fitters aim to keep the overall diameter within 3% of the original. Beyond that, the speedometer error becomes significant, the gearing changes, and on a tight car the tire may rub the arch or suspension at full lock.
Will a larger tire make my speedometer read faster or slower?
Slower. A larger circumference covers more ground per revolution, so for a given true speed the wheels turn fewer times and the speedometer, which counts revolutions, shows a lower number than the real speed.
Does the calculator account for tire pressure or wear?
No. It uses the nominal new-tire dimensions. Inflation changes the rolling radius slightly, and worn tread reduces the diameter by a few millimetres, but neither is large enough to change the buying decision.