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03 / TIRE TOOLS

Speedometer Error Calculator

Estimated speed after a tire change.

GEOMETRY CHECK

Your setup

A / Current
Metric: 225/45R17 · Flotation: 33x12.50R15
Enter separate dimensions
B / New
Metric: 225/45R17 · Flotation: 33x12.50R15
Enter separate dimensions
Input in km/h. The current tire is the calibration reference.

What changes

NOMINAL GEOMETRY

Example calculation. Replace these values with your setup.

Estimated actual speed101.7 km/h

Estimated speed after a tire change

Nominal tire geometry

Estimated speed difference
+1.7 km/h
Speed change from reference
+1.72%

Why your speed changes

Distance in one turn

Current / A
1,992.7 mm
New / B
2,027 mm

The arrows show how far each tire travels in one complete turn. Both drawings use the same scale.

Speed on your dashboard
100 km/h
Estimated road speed
101.7 km/h
Difference
+1.7 km/h

A larger tire travels farther in one turn. With the same dashboard reading, the estimated road speed increases.

ACurrent / A

Current / A: Distance in one turn, 1,992.7 mmThe straight arrow shows the supplied geometric tire circumference. Current and new distances share one scale. The circular arrow represents one wheel turn; it does not measure loaded rolling distance.Distance in one turn1,992.7 mm

BNew / B

New / B: Distance in one turn, 2,027 mmThe straight arrow shows the supplied geometric tire circumference. Current and new distances share one scale. The circular arrow represents one wheel turn; it does not measure loaded rolling distance.Distance in one turn2,027 mm
Orange arrows show the selected measurement. Change your inputs to update the drawing. Values describe static geometry.
Geometry results
MeasurementCurrentNewChange
Indicated speed100.0 km/h100.0 km/h0.0 km/h
Estimated actual speed100.0 km/h101.7 km/h+1.7 km/h
Estimated speed difference——+1.7 km/h
Speed change from reference——+1.72%
Nominal diameter634.3 mm645.2 mm+10.9 mm
Assumptions & limits
  • The reference speedometer is assumed accurate with the current tires. This estimates a change in calibration, not a measured road speed.
  • Markings describe nominal geometry. Actual tire dimensions vary by tire model, measuring rim, pressure, and wear.
  • Unloaded radius and geometric circumference do not measure loaded ride height or actual rolling circumference.
  • Load and speed markings and approved tire-to-rim width combinations are not evaluated by these geometry calculations.

Estimate speed after changing tire size

Use the reference tire, replacement tire and indicated speed to estimate the effect of a circumference change on road speed.

Example · 100 km/h indicated

Changing from 225/45R17 to 235/40R18 gives an estimated actual speed of 101.7 km/h at an indicated 100 km/h, using their nominal circumference ratio.

The estimate assumes that the speedometer was correctly calibrated for the reference tire.

Speedometer questions

Does a larger tire make actual speed higher or lower?

With the same indicated speed and a correctly calibrated reference, a larger rolling circumference means more distance per revolution and a higher estimated actual speed. A smaller circumference has the opposite effect.

Does this include the vehicle’s factory speedometer error?

No. The calculation isolates the nominal tire-size change. Factory calibration bias, wear, pressure and rolling deformation can affect the result; the tool does not measure or calibrate the vehicle.

Can I use miles per hour?

Yes. Imperial displays results in miles per hour. Enter the dashboard speed in km/h; the input unit is shown beside the field.

Methodology & limits

How this tool works

Estimated actual speed = indicated speed × new circumference / reference circumference.