Auto

Calculateur de taille de pneu

Saisissez la taille d’origine et la nouvelle. Obtenez la variation de diamètre et l’erreur du compteur de vitesse.

Calculer

Measurements convert when you switch units

mm
%
in
mm
%
in
km/h

Updates as you type

Résultat

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La formule

Ø = jante×25,4 + 2×(largeur×série/100). Vitesse réelle ≈ indiquée × (Ø_new ÷ Ø_stock).

Ø mm = rim_in × 25.4 + 2 × (width_mm × aspect ÷ 100) · Actual speed ≈ Indicated × (Ø_new ÷ Ø_stock)

Exemple détaillé

  1. Stock 225/45R17 → Ø = 17×25.4 + 2×(225×0.45) = 634.3 mm
  2. New 245/40R18 → Ø = 18×25.4 + 2×(245×0.40) = 653.2 mm
  3. Ratio 653.2÷634.3 ≈ 1.030 · at 100 indicated → ≈ 103.0 actual

Résultat: +3.0% diameter · ~103 at indicated 100

How tire diameter and speedometer error are calculated

We use the industry metric size code. Real rolling radius also depends on load and inflation; this is the geometric overall diameter.

Size code

225/45R17 means 225 mm section width, 45% aspect (sidewall = 45% of width), R = radial, 17-inch rim. Plus-sizing often raises rim size while lowering aspect to keep diameter close.

Speedometer and odometer

Cars assume the stock circumference. Larger tires mean you travel farther per wheel revolution — actual speed is higher than indicated, and the odometer under-reports distance.

What we do not model

No load deflection, wear, or ABS/ESP calibration limits. Many makers advise staying within about ±3% of stock diameter; check your handbook and local rules.

Faits intéressants

Plus-sizing goal

Moving to a larger rim usually pairs with a lower aspect ratio so overall diameter — and gearing — stay near stock.

Legal and warranty

Some regions restrict tire sizes on the vehicle’s placard. Oversized changes can affect speedometer legality and warranty.

Load index matters

Diameter is not enough — match or exceed the load and speed ratings stamped on the sidewall.

AWD caution

On all-wheel-drive cars, mismatched diameters between axles can stress the driveline.

Questions fréquentes

Ø (mm) = rim inches × 25.4 + 2 × (width × aspect/100). Compare stock and new to get the percent change.

Yes. With a larger overall diameter, indicated speed is lower than actual. This tool shows actual speed at your indicated value.

A common rule of thumb is to stay within about 3% of stock diameter, but follow the vehicle maker and tire professional guidance.

Yes — the same circumference factor scales reported distance. Larger tires under-report mileage.

Yes for metric sizes (e.g. 225/45R17). Some light-truck sizes use different notations; convert before using this form.

Références

  1. Tire basics — NHTSA U.S. tire safety and labeling context.
  2. Tyre labelling — European Commission EU tyre label framework.