Speedometer Error Calculator

There is no sensor measuring the road. The vehicle counts pulses from a wheel-speed sensor or a transmission output sensor and converts them using a single constant: the revolutions per mile of the tire that was fitted at the factory. That constant is the only thing tying the count to a distance.

Fit a taller tire and each revolution covers more inches, but the constant does not know. The counted revolutions now represent more ground than the instrument credits, so it under-reads by exactly the proportion that the diameter grew. There is nothing approximate about it — the error is the diameter ratio, to as many decimal places as you care to carry.

1. Original: 265 × 0.70 = 185.5 mm sidewall → 431.8 + 371 = 802.8 mm = 31.606″
2. New: 285 × 0.70 = 199.5 mm sidewall → 431.8 + 399 = 830.8 mm = 32.709″
3. Ratio: 32.709 ÷ 31.606 = 1.03488
4. At an indicated 60 mph: 60 × 1.03488 = 62.09 mph
5. At an indicated 70 mph: 70 × 1.03488 = 72.44 mph
6. Odometer: every 100 miles counted is really 103.49 miles

That is a 3.49% change, which is outside the ±3% rule — one of the reasons this particular upgrade, common as it is on half-ton trucks, is worth doing with the numbers in front of you. See the full 265/70R17 vs 285/70R17 comparison.

Anything the car derives from distance or wheel speed. The odometer and trip computer under-count. Fuel economy computed from those miles reads low, so a truck that is genuinely doing 20 mpg reports about 19.3 on a 3.49% oversize. Service intervals arrive late. Cruise control holds a true speed slightly above the set figure.

Safety systems are the part worth taking seriously. ABS and stability control compare wheel speeds against each other, so a uniform change across all four tires is largely invisible to them — but a mismatched pair, or a spare of a different diameter, is not.

How do you calculate speedometer error from tire size?

Divide the new overall diameter by the old one and multiply the indicated speed by the result. Going from 31.61 inches to 32.71 inches gives a ratio of 1.0349, so an indicated 60 mph is really 62.1 mph.

Do bigger tires make the speedometer read high or low?

Low. A taller tire covers more ground per revolution, so the same number of counted revolutions represents more distance than the instrument assumes. You are going faster than it says, and your odometer is counting fewer miles than you drove.

How much speedometer error is legal?

Rules vary, but the widely used engineering guideline is the same ±3% of tire diameter used for substitution. Most manufacturers deliberately bias speedometers to read slightly high from the factory, which means a modest oversize can bring the reading closer to true rather than further away.

Does a tire size change affect the odometer too?

Yes, by exactly the same percentage, because both are derived from the same revolution count. A 3.49% taller tire means every 100 miles shown is really 103.5 miles — over 60,000 indicated miles, more than 2,000 unrecorded.

Can the speedometer be recalibrated?

On most vehicles built since the late 1990s the correction is a software value for revolutions per mile, set with a scan tool or a calibration module. On older cable-driven speedometers it means changing a physical drive gear in the transmission.