RimRatio

Speedometer error calculator

Changed tire size? Your speedometer is still counting wheel revolutions for the old tire. Enter both sizes to see your true speed at every indicated speed.

255/85R16 is 2.60 in (66 mm) taller than 245/75R16 (+8.5%). With a speedometer calibrated for 245/75R16, an indicated 60 mph is really 65.1 mph. The diameter difference is more than ±3%, a range often quoted as a rule of thumb, so speedometer, gearing and clearance effects are larger.

Geometry only: this does not confirm that a size fits, is approved for or is safe on a particular vehicle. Wheel width and offset, suspension, clearances, load index, speed rating, the vehicle maker's specification and local regulations all apply — check with your owner's manual and a qualified tire professional.

Indicated vs true speed (mph)
Speedometer showsTrue speed on 255/85R16Difference
20 mph21.7 mph+1.7 mph
30 mph32.6 mph+2.6 mph
40 mph43.4 mph+3.4 mph
50 mph54.3 mph+4.3 mph
60 mph65.1 mph+5.1 mph
70 mph76.0 mph+6.0 mph
80 mph86.8 mph+6.8 mph
Indicated vs true speed (km/h)
Speedometer showsTrue speed on 255/85R16Difference
30 km/h32.6 km/h+2.6 km/h
50 km/h54.3 km/h+4.3 km/h
80 km/h86.8 km/h+6.8 km/h
100 km/h108.5 km/h+8.5 km/h
120 km/h130.2 km/h+10.2 km/h

Assumes the speedometer was accurate on 245/75R16. Factory speedometers often read slightly high on purpose; check against GPS.

The formula

true speed = indicated speed × (new diameter ÷ original diameter)

For 245/75R16 (30.47 in) to 255/85R16 (33.07 in) the factor is 1.0853: true speed is 8.53% higher than indicated, i.e. the speedometer reads about 7.86% low.

The same factor applies to the odometer and to any trip-computer distance or fuel-economy figure.

Frequently asked questions

Do bigger tires make my speedometer read faster or slower?

Slower. A taller tire covers more ground per revolution, but the speedometer still counts revolutions as if the original tire were fitted. With 255/85R16 instead of 245/75R16, an indicated 60 mph is really 65.1 mph.

Does the odometer change too?

Yes, by the same percentage. With 255/85R16 instead of 245/75R16 the odometer records about 7.9 miles too few per 100 miles actually driven, which also affects fuel-economy calculations.

Can I recalibrate the speedometer?

On many vehicles a dealer or a programming tool can store a new tire size or revolutions-per-mile value; older mechanical speedometers used a replaceable driven gear. What is possible depends on the vehicle.

Why does my speedometer read high even with stock tires?

Speedometers commonly read slightly high: UN Regulation No. 39, applied in many countries, does not allow a speedometer to show less than the true speed. Worn tread also makes the tire slightly shorter. Compare against GPS to know your actual starting point.

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