Tire Size Calculator & Decoder
Decode a tire size like 205/55R16 into everything it means: section width, aspect ratio, sidewall height, overall diameter, circumference and how many times it turns per mile and kilometre. Enter a second size to compare — useful when changing tire sizes, since a different overall diameter throws off your speedometer and odometer. The tool shows how far off the reading would be.
How to use this tool
- Enter a tire size in the standard format, such as 205/55R16.
- Read the decoded specification: width, aspect ratio, sidewall, overall diameter, circumference and revolutions per mile and kilometre.
- To compare, enter a second size in the optional box.
- Check the diameter difference and speedometer effect — within ±3% is generally considered acceptable.
- A larger overall diameter makes your true speed higher than the speedometer shows, and a smaller one the reverse.
The formula
A tire code gives the section width in millimetres, the aspect ratio (the sidewall as a percentage of the width) and the wheel diameter in inches. From those, the geometry follows directly.
sidewall = width × aspect ÷ 100
overall diameter = wheel (in mm) + 2 × sidewall
circumference = π × diameter
revs per mile = 1,609,344 ÷ circumference (mm)- section width
- Tread width in millimetres (the first number)
- aspect ratio
- Sidewall height as a percent of width (the second number)
- R
- Radial construction; the number after it is wheel diameter in inches
Keeping the overall diameter within about 3% of the original preserves speedometer accuracy, gearing and clearance.
Worked examples
Decoding a common size
- Given
- 205/55R16
- Result
- ≈ 631 mm overall diameter
Width 205 mm, sidewall 55% of that (113 mm), on a 16-inch (406 mm) wheel: 406 + 2×113 ≈ 631 mm.
A plus-size fitment
- Given
- 205/55R16 vs 225/45R17
- Result
- Within a couple of percent
Going wider and lower on a bigger wheel can keep the overall diameter nearly the same — the goal of “plus sizing”.
Speedometer effect
- Given
- A tire 3% larger in diameter
- Result
- Speedo reads ~3% low
At an indicated 60, you would actually be doing about 62, because each revolution covers more ground.
Frequently asked questions
The 205 is the section width in millimetres. The 55 is the aspect ratio — the sidewall height is 55% of the width. The R means radial construction, and the 16 is the wheel diameter in inches. Together they define the tire’s size and shape.
The speedometer is calibrated to the original tire’s rolling circumference. A larger overall diameter travels further per revolution, so your true speed is higher than shown; a smaller diameter does the reverse. The tool shows the percentage difference and the true speed at an indicated 60.
A common guideline is to stay within about 3% of the original overall diameter. Beyond that, the speedometer error grows, and you risk clearance problems, altered gearing and, on some vehicles, issues with traction and stability systems. Always check clearance at full steering lock and suspension travel.
Fitting a larger-diameter wheel with a lower-profile tire so the overall diameter stays about the same. It sharpens handling and looks and keeps the speedometer accurate, but the shorter sidewall gives a firmer ride and less protection from potholes.
How many times the tire turns to cover a mile (or kilometre). It reflects the rolling circumference and is what speedometers and odometers actually count. Two tires with different revs per mile will produce different speed and distance readings on the same vehicle.
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