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Tire size & speedometer error calculator

Changing wheel or tire size? Compare old vs. new tire diameter to see exactly how far off your speedometer and odometer will read.

Old (stock) tire

New tire

—old diameter
vs
—new diameter
—actual speed
—speedometer error
—revs/mile change

🔒 Runs entirely in your browser — nothing is sent anywhere.

Why "plus sizing" throws off your speedometer

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Diameter, not size label

A tire's size code (width/aspect/wheel) determines its overall diameter — that diameter, not the label itself, is what actually changes your speed reading.

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Bigger reads slower

A larger overall diameter covers more ground per revolution, so your speedometer under-reports your real speed — you're going faster than it says.

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Odometer drifts too

Trip computers and odometers use the same signal, so long trips will read shorter (bigger tires) or longer (smaller tires) than actual distance driven.

Tire & speedometer FAQ

Why does changing tire size affect my speedometer?

Your speedometer is calibrated to the factory tire's revolutions per mile. A larger tire covers more ground per revolution, so you're actually going faster than shown. A smaller tire covers less ground, so actual speed is slower than indicated.

How do I read a tire size like 225/45R17?

225 is the section width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 17 is the wheel diameter in inches.

Does this affect my odometer too?

Yes — the odometer uses the same wheel-revolution signal, so it's off by the same percentage: under-reporting distance with larger tires, over-reporting with smaller ones.