Speedometer Error Calculator
See how a change in tire diameter throws off your speedometer, and find your true speed after fitting taller or shorter tires.
How it works
True speed = indicated × (new diameter ÷ old diameter) · Error% = (new − old) ÷ old × 100
Your speedometer counts how fast the wheels turn and assumes the factory tire diameter to convert that into speed. Fit a taller tire and each revolution covers more ground, so you are actually going faster than the dash shows (it under-reads); a shorter tire does the opposite. The error is simply the ratio of the new to old rolling diameter. This is why lift-kit and off-road tire changes usually require a speedometer recalibration, and why staying within about 3% of the original diameter keeps the reading legal and the ABS behaving.
Worked example
Stock tires are 24.97 in and you fit 26.5 in — a +6.13% increase. At an indicated 60 mph you are truly doing 60 × (26.5 ÷ 24.97) ≈ 63.7 mph, so the speedometer under-reads by about 3.7 mph.
Frequently asked questions
Do taller tires make my speedometer read fast or slow?
Slow. A taller tire travels farther per revolution, so at a true 60 mph the wheels spin more slowly than the stock size would and the speedometer shows less than your real speed. You are actually going faster than the dash indicates.
How much tire-size change is safe before recalibrating?
Staying within roughly 3% of the original diameter keeps speedometer error small and usually avoids ABS or transmission issues. Bigger changes — common with lift kits and off-road tires — warrant a recalibration or a programmer.
Does this affect my odometer too?
Yes. The odometer uses the same wheel-revolution count, so taller tires make it record fewer miles than you actually drive, and shorter tires overstate mileage — worth knowing for warranty, resale and service intervals.
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Educational estimate — real figures vary by vehicle, load, conditions and manufacturer specs. Always follow your owner’s manual.