TIRE & WHEEL LAB
Speedometer Error Calculator
See how a change in rolling diameter affects speed readings, assuming the original tire was correctly calibrated.
Your results
LIVE CALCULATIONResults will appear here when JavaScript loads. See the worked example below for a reference calculation.
How this calculator works
Actual speed = indicated speed × new tire diameter ÷ original tire diameter. Change (%) = (new diameter ÷ original diameter − 1) × 100.
Read the full methodology for definitions, conversion factors and limitations.
Worked example
Changing 205/55 R16 to 215/55 R16 gives a 1.0174 speed multiplier. At an indicated 100 km/h the theoretical road speed is 101.74 km/h.
Before you act on the result
Tire and wheel dimensions cannot confirm fitment. Check wheel width, offset, suspension and wheel arch clearance, load and speed ratings, and manufacturer recommendations. Ask a qualified tire professional to verify modifications. Actual tire measurements vary with construction, pressure, load and wear.
Frequently asked questions
Does this include the car’s factory speedometer error?
No. It isolates the additional geometric effect of the tire change. Factory calibration and tire wear can alter real readings.
Does the odometer change too?
The same change in distance per wheel revolution can affect recorded distance, unless the vehicle is recalibrated.