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Tire Size & Speedometer Error Calculator

Compare two tire sizes by their code (e.g. 225/45R17) and see the resulting change in speedometer and odometer accuracy.

Original tire diameter
24.97 in
New tire diameter
25.72 in
Diameter change
+2.98%
Original revolutions per mile
807.6
New revolutions per mile
784.2
Actual speed
61.8 mph (speedometer shows 60 mph)
Odometer error
+2.98%

How it works

A tire's code (like 225/45R17) packs in its geometry: the sidewall height is the width times the aspect ratio (225mm × 45% = 101.25mm), and the overall diameter is the rim diameter plus two sidewalls, converted to a single unit — the standard tire-industry formula, verified against multiple tire-size references rather than derived from memory. Circumference follows from diameter × π, and dividing the fixed 63,360 inches in a mile by that circumference gives revolutions per mile.

A car's speedometer and odometer are calibrated to how many wheel revolutions the factory tire makes per mile. Fit a physically larger tire and each revolution covers more real distance than the electronics assume, so both the displayed speed and the accumulated mileage read low relative to the truth — a smaller replacement tire has the opposite effect. This calculator scales your entered speedometer reading by the ratio of the two tires' diameters to estimate your actual road speed.

The percentage change in diameter and the odometer error are the same number: whatever fraction bigger (or smaller) the new tire's diameter is, that's how far off the odometer and speedometer both read, since both derive from the same revolutions-per-mile figure.

FAQ

Why is there no metric/imperial toggle for the tire dimensions?

A tire size code is an internationally standardized format that always mixes millimeters (width), a unitless percentage (aspect ratio), and inches (rim diameter) — that's simply how tires are labeled worldwide, not a measurement this site's users would want converted to a different system.

Does a bigger tire always mean my speedometer under-reads my real speed?

Yes, assuming nothing else about the car's calibration changes: a larger-diameter tire covers more ground per revolution than the speedometer's factory calibration expects, so the display shows a lower speed (and lower mileage) than what's actually happening. A smaller-diameter replacement does the reverse, over-reading both speed and mileage.

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