Why does changing tyre size affect speedometer accuracy?

Speedometers count wheel rotations and multiply by the circumference (π × diameter) to calculate distance travelled. A larger-diameter tyre covers more ground per rotation, so the vehicle moves faster than the speedometer indicates. A smaller tyre covers less ground, causing the speedometer to over-read actual speed.

What does 'speedo error' percentage mean?

Speedo error is the percentage difference between indicated and actual speed. A +2.85% error means the speedometer under-reads: when it shows 70 mph, true speed is 72 mph. A negative error means the speedometer over-reads. Zero error means the reading is accurate for that tyre size.

Does this calculation work for both metric and imperial units?

Yes. The diameter ratio cancels units, so the speed correction applies identically whether indicated speed is in mph or km/h. Tyre diameters must be entered in the same unit (this tool uses millimetres) but the ratio itself is dimensionless.

Can I use tyre sidewall markings to find diameter?

Tyre markings show width, aspect ratio, and rim diameter (e.g. 205/55 R16). Overall diameter = rim diameter + 2 × (width × aspect ratio ÷ 100). For 205/55 R16: 16 inches + 2 × (205 mm × 0.55) = 406.4 mm + 225.5 mm ≈ 632 mm. Convert inches to millimetres (×25.4) before adding.

Does tyre pressure affect the result?

Yes. Under-inflation compresses the tyre, reducing effective rolling diameter and making the speedometer over-read slightly. This calculator assumes both the original and new tyres are inflated to their respective manufacturer specifications. Pressure-induced diameter changes are typically small (1–2%) but measurable at very low pressures.

Performance & Engineering

Tyre Size Effect on Speedo

Calculate your true speed when fitting different-diameter tyres to your car.

Last updated: 25 June 2026

What this tool does

This calculator computes actual road speed versus indicated speedometer reading when tyre diameter changes from the original equipment specification. It uses the direct-proportion relationship between tyre diameter and distance-per-revolution: actual speed = indicated speed × (new diameter ÷ original diameter). Outputs include true speed, speedo error percentage, and direction of error (under-read or over-read). The method assumes the speedometer was correctly calibrated for the original tyres and that both tyres rotate at identical RPM.

Inputs

Result

—

Formula

v_{actual} = v_{indicated} \times \frac{d _{new}}{d _{OE}}

v_{actual}

True vehicle speed (mph)

v_{indicated}

Speedometer reading (mph)

d_{new}

New tyre diameter (mm)

d_{OE}

Original equipment tyre diameter (mm)

How tyre size speedo accuracy calculation works

Vehicle speedometers measure wheel rotations and convert them to speed based on the original-equipment tyre diameter programmed into the vehicle's computer. When a different-diameter tyre is fitted, each wheel rotation covers a different distance, so the speedometer reading no longer matches true road speed. This calculator applies the diameter ratio to compute actual speed for any given speedometer reading.

The formula

Actual speed = Indicated speed × (New tyre diameter ÷ Original tyre diameter)

Speedo error (%) = [(New diameter ÷ Original diameter) − 1] × 100

Where all diameters are overall tyre diameter in millimetres. A positive error means the speedometer under-reads (true speed is higher); a negative error means it over-reads (true speed is lower).

Where this method is most accurate

The calculation is exact for the mechanical relationship between tyre diameter and distance travelled per revolution, assuming both tyres rotate at the same speed and the speedometer was correctly calibrated for the original tyres. Real-world accuracy depends on tyre pressures remaining at specification (under-inflation reduces effective diameter), consistent tread depth across both tyres, and the absence of transmission slip or wheel spin. The method does not account for tyre deformation under load or variations in rolling radius due to sidewall stiffness.

What this tool does not do

This calculator does not verify whether a given tyre size is legal, safe, or physically fits a specific vehicle. It does not adjust for differences in tyre width, load rating, speed rating, or changes to suspension geometry. The result shows speedo error magnitude but does not determine whether that error meets regulatory requirements in any jurisdiction. The tool does not measure actual tyre diameter; users must obtain diameter values from tyre specifications or direct measurement.

Disclaimer

This tool is for educational and comparison purposes only. It performs a mathematical calculation and does not constitute vehicle-safety advice, roadworthiness certification, or legal compliance guidance. Actual vehicle speed may differ due to tyre wear, pressure, temperature, load, and calibration variances. Always verify tyre fitment with the vehicle manufacturer and comply with local laws governing speedometer accuracy and tyre specifications.

Questions

Why does changing tyre size affect speedometer accuracy?: Speedometers count wheel rotations and multiply by the circumference (π × diameter) to calculate distance travelled. A larger-diameter tyre covers more ground per rotation, so the vehicle moves faster than the speedometer indicates. A smaller tyre covers less ground, causing the speedometer to over-read actual speed.
What does 'speedo error' percentage mean?: Speedo error is the percentage difference between indicated and actual speed. A +2.85% error means the speedometer under-reads: when it shows 70 mph, true speed is 72 mph. A negative error means the speedometer over-reads. Zero error means the reading is accurate for that tyre size.
Does this calculation work for both metric and imperial units?: Yes. The diameter ratio cancels units, so the speed correction applies identically whether indicated speed is in mph or km/h. Tyre diameters must be entered in the same unit (this tool uses millimetres) but the ratio itself is dimensionless.
Can I use tyre sidewall markings to find diameter?: Tyre markings show width, aspect ratio, and rim diameter (e.g. 205/55 R16). Overall diameter = rim diameter + 2 × (width × aspect ratio ÷ 100). For 205/55 R16: 16 inches + 2 × (205 mm × 0.55) = 406.4 mm + 225.5 mm ≈ 632 mm. Convert inches to millimetres (×25.4) before adding.
Does tyre pressure affect the result?: Yes. Under-inflation compresses the tyre, reducing effective rolling diameter and making the speedometer over-read slightly. This calculator assumes both the original and new tyres are inflated to their respective manufacturer specifications. Pressure-induced diameter changes are typically small (1–2%) but measurable at very low pressures.

Spotted something off?

Calculations or display — let us know.

Sources & Methodology

The calculator applies the direct-proportion relationship between tyre diameter and road speed: actual speed = indicated speed × (new diameter ÷ original diameter). Speedometer error percentage is computed as [(new ÷ original) − 1] × 100. The method assumes the speedometer was factory-calibrated for the original tyre size and that wheel RPM remains constant.

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