Tire Size Calculator
Decode a tire size like 225/45R17 into its overall diameter, circumference and revolutions per mile, and compare two sizes for speedometer error.
How to use this calculator
- 1Enter a tire size exactly as printed on the sidewall, e.g. 225/45R17.
- 2Turn on comparison to check how a different size would affect your speedometer and odometer accuracy.
How the calculation works
Sidewall height = Width × Aspect ratio. Diameter = Wheel diameter + 2 × Sidewall height. Circumference = Diameter × π- Width
- Tire width in millimetres — the first number, e.g. 225
- Aspect ratio
- Sidewall height as a percentage of width — the second number, e.g. 45 means 45%
- Wheel diameter
- The rim size in inches — the number after "R"
The sidewall appears twice in the diameter formula because it wraps both above and below the wheel — the tire adds its height on top of the rim and again underneath.
Speedometers and odometers are calibrated for a specific tire circumference; changing tire size without recalibrating shifts both readings by the same percentage as the diameter change.
Worked example
225/45R17
- 1.Sidewall height: 225 × 0.45 = 101.25 mm.
- 2.Diameter: 17 + (2 × 101.25 ÷ 25.4) = 17 + 7.97 = 24.97 in.
- 3.Circumference: 24.97 × π ≈ 78.45 in.
Result: 24.97 in diameter, 78.45 in circumference
Reading the code on a tire's sidewall
A size like 225/45R17 packs three separate measurements into one short code, each in a different unit, which is exactly why it looks cryptic at first glance. The first number (225) is the tire's width in millimetres, measured sidewall to sidewall. The second number (45) is the aspect ratio — the sidewall's height expressed as a percentage of that width, so a 225 mm-wide tire with a 45 aspect ratio has a sidewall 45% of 225 mm tall. The letter R means radial construction, the dominant type on virtually every modern passenger vehicle. The final number (17) is the wheel diameter in inches, which the tire has to match exactly to fit the rim.
Mixing millimetres, a percentage, and inches in a single code is a historical accident rather than a deliberate design — the format accumulated its parts as tire technology and international standards evolved separately, and by the time anyone might have cleaned it up, it was too entrenched in manufacturing and retail to change.
Why overall diameter is the number that actually matters
None of the three numbers on a tire's sidewall tells you its overall size on its own — you need to combine all three to get the outside diameter, which is what actually determines how the tire fits your vehicle, how far it rolls per revolution, and how accurate your speedometer stays. Two completely different-looking size codes can produce nearly the same overall diameter, and two sizes that look superficially similar can differ meaningfully. This is the entire reason "plus sizing" — fitting a wider tire on a larger wheel while keeping the overall diameter close to stock — works at all.
- Width — affects grip and appearance most directly, but by itself says nothing about overall size.
- Aspect ratio — a lower number means a shorter, stiffer sidewall relative to the tire's width — common on performance-oriented setups — while a higher number gives a taller, more cushioned sidewall.
- Wheel diameter — must match the rim exactly; this is the one number in the code with zero tolerance for substitution.
What changing tire size actually does to the vehicle
A speedometer and odometer are calibrated for the factory tire's rolling circumference — how far the vehicle travels per wheel revolution. Fit a tire with a larger overall diameter and the vehicle actually travels farther per revolution than the speedometer assumes, so the speedometer under-reads: it will show 60 mph when the car is really going slightly faster. A smaller-diameter tire does the opposite, making the speedometer over-read. The size and direction of this error is exactly what the diameter-difference percentage in this calculator estimates.
The same rolling-circumference change also affects the odometer, fuel-economy readouts, and on many modern vehicles the ABS and traction-control systems, which rely on wheel-speed sensors calibrated to the stock tire size.
Common pitfalls when comparing or changing sizes
A few things trip people up when sizing tires or interpreting the numbers.
- Nominal size vs measured size — the size printed on a tire is a nominal figure — actual manufactured dimensions vary slightly by brand and model even for an identical marked size, sometimes called "plus sizing" tolerance.
- A small percentage change compounds — a diameter change that looks minor on paper (a few percent) still shifts speedometer, odometer and sometimes traction-control accuracy by that same percentage — small enough to ignore for a couple of percent, but worth recalibrating for anything larger.
- Width and diameter are independent — going wider does not automatically mean going taller — width, aspect ratio and wheel diameter can each change somewhat independently, which is exactly why the aspect ratio exists as its own number in the code rather than being fixed.
What this assumes, and where it stops
Assumptions
- The tire is inflated to its rated pressure — under- or over-inflation changes the actual rolling diameter slightly from this nominal calculation.
Limitations
- This uses the nominal size printed on the tire, not the measured "plus sizing" real-world diameter, which can vary slightly between manufacturers for the same marked size.
Common questions
Why would I want a different tire size than stock?
Common reasons include fitting a different wheel diameter for looks or performance, matching a specific tire's availability, or slightly altering gearing — but any change to overall diameter affects speedometer accuracy, ground clearance and sometimes fender clearance, so it is worth checking against the stock size first.
How much tire size difference is safe?
Most tire professionals suggest keeping the overall diameter change within about 3% of stock to avoid meaningfully affecting speedometer accuracy, ABS/traction control calibration, and clearance — bigger changes are done deliberately (often with a speedometer recalibration) rather than incidentally.
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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