Speedometer Gear Calculator — Tyre Size Correction
When you fit bigger or smaller tyres, your speedometer reads incorrectly because it was calibrated for the original tyre diameter. Enter the old and new tyre sizes in the standard format (e.g. 225/55R17 → width 225 mm, aspect 55 %, rim 17 in) and the current speedometer reading to find your actual speed and the error percentage.
Speedometer unit
km/h
mm
%
in
mm
%
in
Speedometer overreads — you are going slower than it shows
- 1
Old tyre diameter
2 × (225 × 55 ÷ 100) + 17 × 25.4 = 679.3Section width × aspect ratio / 100 gives sidewall height; rim inches × 25.4 converts to mm. - 2
New tyre diameter
2 × (245 × 45 ÷ 100) + 18 × 25.4 = 677.7 - 3
Correction factor
677.7 ÷ 679.3 = 0.99764 - 4
Actual speed
100 × 0.99764 = 99.8
How does this calculator work?
When tyre size changes, actual speed = indicated speed × (new tyre diameter / old tyre diameter). Tyre diameter (mm) = 2 × (section_width × aspect_ratio/100) + rim_diameter_in × 25.4. Bigger tyres → speedometer underreads; smaller tyres → overreads. A ~3 % diameter change causes ~3 % speed error.
Formula
How this is calculated
A car speedometer measures wheel rotation speed — in modern vehicles via a wheel speed sensor, in older vehicles via a mechanical cable driven from a gear on the transmission or differential. The speedometer translates a given number of wheel rotations per second into a displayed speed, using the factory tyre's diameter as its reference. When you fit a tyre with a different overall diameter, every revolution covers a different distance: a larger tyre covers more ground per revolution, so at any given wheel RPM you actually travel faster than the display shows (underreading); a smaller tyre means the opposite (overreading).
The correction is simple: actual_speed = indicated_speed × (new_diameter / old_diameter). The overall tyre diameter from the standard tyre code (e.g. 225/55R17) is: diameter_mm = 2 × (section_width_mm × aspect_ratio/100) + rim_diameter_in × 25.4. The section width is the tyre's width in mm; the aspect ratio is the sidewall height as a percentage of the section width; and the rim diameter is in inches, converted by multiplying by 25.4.
A 3–4 % error (typical when "plus-sizing" by one step) is enough to receive a speeding ticket or fail an MOT/roadworthy test in many countries. In the UK, the speedometer must never underread but may overread by up to 10 %. Checking your GPS speed against the corrected figure is the most accurate real-world confirmation. Older vehicles with cable-driven speedometers can be corrected by fitting a different plastic driven gear inside the transmission; the number of teeth required scales with the change in tyre revolutions per km.
Frequently asked questions
Not necessarily — it depends on the overall tyre diameter, not the rim size alone. If you fit larger rims with lower-profile tyres (reduced aspect ratio), the overall diameter may stay almost the same. The speedometer error depends entirely on the change in overall diameter: larger diameter → speedometer underreads; smaller diameter → speedometer overreads.
Most manufacturers recommend staying within ±3 % of the original overall diameter to keep speedometer error within legal and safety limits. A change greater than ~5 % is generally noticeable and may require a speedometer correction module or replacement driven gear in cable-driven systems.
Cable-driven speedometers use a plastic driven gear inside the transmission whose tooth count sets the calibration. To correct for a tyre change, calculate new_teeth = old_teeth × (new_diameter / old_diameter). Replacement driven gears in the correct tooth count are available from specialist suppliers for most classic vehicles.
Also known as
TG we-Calculate Editorial Team. (2026). Speedometer Gear Calculator — Tyre Size Correction [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/speedometer-gear-calculator
TG we-Calculate Editorial Team. "Speedometer Gear Calculator — Tyre Size Correction." TG we-Calculate. 2026. https://we-calculate.com/calculator/speedometer-gear-calculator.
TG we-Calculate Editorial Team, "Speedometer Gear Calculator — Tyre Size Correction," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/speedometer-gear-calculator
@misc{wecalculate_speedometer_gear_calculator, title = {Speedometer Gear Calculator — Tyre Size Correction}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/speedometer-gear-calculator}}, year = {2026}, note = {TG we-Calculate} }
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