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
What Tire Size Actually Means
A tire size like 205/55R16 reads as: 205 mm section width, 55% aspect ratio (sidewall height as a percentage of width), mounted on a 16-inch rim. From these three numbers the tool computes the tire's overall diameter and circumference.
Comparing two sizes shows the diameter difference — the single most important number when fitting a non-stock size. A difference beyond roughly ±3% is generally considered enough to throw off your speedometer, odometer and ABS/traction-control calibration noticeably; many tire shops treat ±3% as the safe ceiling for a "plus-sizing" swap.
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, a bigger tire makes your actual speed higher than what the dashboard shows — and a smaller tire does the reverse.
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, ABS and clearance effects become more noticeable.
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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Tire Size Calculator & Comparator