Tire Size Calculator & Comparator
Enter a tire size (e.g. 205/55R16) to get its overall diameter and circumference, or compare two sizes to see the speedometer error and diameter difference.
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Diameter difference exceeds 3% — speedometer, ABS and body clearance may be noticeably affected. Confirm with a tire professional before fitting.
Speedometer Error
How Tire Size Is Calculated
A size like 205/55R16 encodes three numbers: 205 is the section width in millimeters, 55 is the aspect ratio — the sidewall height as a percentage of that width — and R16 means it mounts on a 16-inch rim. From these, sidewall height = 205 × 0.55 = 112.75mm, rim diameter = 16 × 25.4 = 406.4mm, and overall diameter = rim diameter + 2 × sidewall height = 406.4 + 225.5 = 631.9mm. Circumference follows directly: π × diameter ≈ 1,985mm, or roughly 1.985m per wheel revolution.
Comparing sizes matters because swapping to a non-stock size changes that circumference. Take 205/55R16 (631.9mm) against a 215/60R16 mounted on the same rim: sidewall height becomes 215 × 0.60 = 129mm, overall diameter becomes 406.4 + 258 = 664.4mm — a 5.1% increase. That single number cascades into two separate real-world effects: the speedometer, which estimates speed from wheel rotation count using the stock tire's circumference, now under-counts — your actual speed runs higher than what the dashboard shows, because each rotation covers more real ground than the ECU assumes. The odometer suffers the identical error in the same direction: it under-reports distance traveled, since it also counts rotations rather than measuring ground covered directly. Most manufacturers and tire shops treat ±3% overall-diameter difference as the safe ceiling for a "plus-sizing" swap — 5.1% is enough to be noticeable on both the speedometer and the odometer, and can also throw off ABS and traction-control calibration, which rely on wheel-speed sensors tuned to the stock rotation-to-distance ratio.
What to Know
The geometry here — section width × aspect ratio × rim diameter — is exact; it is the same formula tire manufacturers use to print load and speed ratings. What it cannot know is your specific car: wheel offset, suspension travel and body clearance all affect whether a given size physically fits without rubbing, and none of that is encoded in the size string itself. A smaller-than-stock tire produces the mirror-image error — the speedometer and odometer both read slightly high, because each wheel rotation covers less ground than the ECU assumes. Small aspect-ratio and larger rim-diameter combinations are frequently chosen specifically to keep overall diameter within that ±3% window (going up in rim size while dropping aspect ratio, commonly called "plus-sizing"), which is why a 205/55R16 and a 225/45R17 can end up almost identical in overall diameter despite looking very different on paper. Always confirm a non-stock fitment with a tire professional before buying.
Frequently Asked Questions
Why does a bigger tire make my speedometer read low?
A larger overall diameter covers more ground per wheel rotation. Since the speedometer estimates speed from rotation count using the stock tire's circumference, a bigger tire makes your actual speed higher than what the dashboard shows — a smaller tire does the reverse.
Does a different tire size throw off the odometer too?
Yes, in the same direction as the speedometer. The odometer also counts wheel rotations rather than measuring distance directly, so a larger tire makes it under-report the distance you actually drove, and a smaller tire makes it over-report.
What counts as a "safe" size difference?
There's no universal legal threshold, but ±3% overall diameter is the widely-used rule of thumb tire shops apply before flagging a fitment as needing a closer look — beyond that, speedometer, odometer, ABS and clearance effects become more noticeable.
Why can two very different-looking sizes end up with almost the same diameter?
Going up a rim size while dropping the aspect ratio — "plus-sizing" — is a deliberate way to keep overall diameter close to stock while changing the tire's profile and wheel appearance. A 205/55R16 and a 225/45R17, for example, can land within a percent of each other despite very different numbers, because the larger rim is offset by the shorter sidewall.
Is this exact enough to certify a fitment?
The math (section width × aspect ratio × rim diameter) is exact geometry, but real-world clearance also depends on wheel offset, suspension travel and body clearance, which this tool doesn't know — always confirm a non-stock fitment with a tire professional.
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