Calculateur de Dimensions de Pneus

Comparez des dimensions de pneus : diamètre, flanc, largeur, écart du compteur et montage

Comparez des dimensions de pneus

Saisissez votre dimension actuelle et jusqu'à deux nouvelles pour voir diamètre, flanc, écart du compteur et montage

Dernière mise à jour: octobre 2, 2026

How to Read a Tire Size

Every metric tire size packs three dimensions into one code. Take 265/70R17, the most common light-truck size in the US:

265: section width

The width in millimetres, sidewall to sidewall, mounted on the recommended rim. 265 mm is 10.4 inches. The tread is narrower than this.

70: aspect ratio

The sidewall height as a percentage of the width, not a measurement. 70% of 265 mm is a 185.5 mm (7.3 in) sidewall. Same number, wider tire, taller sidewall.

R17: wheel diameter

R is radial construction; 17 is the wheel diameter in inches. A tire only fits a wheel of exactly this diameter.

The mix of millimetres, a percentage and inches in one code is why tire sizes are hard to compare by eye. Overall diameter is the wheel plus two sidewalls: 17 in + 2 × 7.3 in = 31.6 in. A prefix of P (passenger) or LT (light truck) changes the load construction, not these dimensions.

The Formulas

  • Sidewall height = width × aspect ratio ÷ 100 (mm)
  • Overall diameter = wheel diameter × 25.4 + 2 × sidewall (mm)
  • Circumference = π × diameter
  • Revolutions per mile = 63,360 ÷ circumference in inches
  • True speed = indicated speed × new diameter ÷ original diameter

These are the nominal design dimensions every manufacturer specifies from. A real tire can measure a few tenths of an inch differently depending on the model, tread depth and inflation, so treat results as the size you are buying, not a caliper reading.

The 3% Rule for Changing Size

Most tire shops and manufacturers treat about 3% of overall diameter as the comfortable limit for a size swap. Inside that band the speedometer error is small, the anti-lock and stability systems behave as designed, and clearance is rarely a problem.

Diameter changeWhat to expect
Up to ±3%Usually a straight swap. Speedometer off by under 2 mph at 60.
3% to 5%Check clearance at full lock and full compression. Speedometer error becomes noticeable.
Over 5%Typically needs a leveling kit or lift, speedometer recalibration, and on trucks often a re-gear.

Diameter is only half of it. A wider tire also needs a wheel inside its approved rim-width range, and room at the strut and inner fender. The calculator shows how far each sidewall moves out or in.

Plus Sizing: Bigger Wheels, Same Diameter

"Plus one" or "plus two" sizing fits a larger wheel while keeping the overall diameter the same by lowering the aspect ratio. 225/65R17 (28.5 in) and 225/55R19 (28.7 in) are within 1% of each other, so the speedometer barely changes, but the 19-inch setup has a much shorter sidewall: sharper steering, a firmer ride and more risk of wheel damage from potholes. Compare both in the calculator before committing to new wheels.

What the Size Does Not Tell You

The size code says nothing about how much weight a tire can carry or how fast it can safely run. Those are the load index and speed rating printed after the size (for example 116T), and a replacement must meet or exceed the figures on your vehicle's door-jamb placard. LT tires also carry a load range (C, D, E) that matters for towing. Match those first; then use the size to fine-tune height and width.

Comment nous calculons cela

Méthode
Nominal tire dimensions from the metric size code
Formule
Sidewall (mm) = width x aspect / 100. Overall diameter (mm) = rim (in) x 25.4 + 2 x sidewall. Circumference = pi x diameter. Revolutions per mile = 63,360 / circumference (in). True speed = indicated speed x new diameter / original diameter. Ground clearance change = (new diameter - original diameter) / 2.
Source
The metric tire size designation (ISO 4000 / ETRTO, used by every P-metric, LT-metric and Euro-metric tire): section width in millimetres, aspect ratio as a percentage of width, rim diameter in inches.
Limites
Results are nominal design dimensions. A real tire can measure a few tenths of an inch differently by model, tread depth and inflation, and a loaded tire rolls slightly smaller, so real revolutions per mile run a little higher. The 3% band is a widely used rule of thumb, not a guarantee of fit: clearance, rim width, load index and speed rating must be checked separately. Flotation sizes (e.g. 33x12.50R15) are not parsed.
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Foire Aux Questions

265 is the section width in millimetres. 70 is the aspect ratio: the sidewall is 70% as tall as the tire is wide, so 185.5 mm. R means radial construction, and 17 is the wheel diameter in inches. Overall diameter is the wheel plus two sidewalls: 17 + 2 x 7.3 = 31.6 inches. A P or LT in front marks a passenger or light-truck tire, which matters for load capacity but not for these dimensions.

The common rule of thumb is to stay within about 3% of the original overall diameter. Inside that band, speedometer error stays small and most vehicles have the clearance. Beyond it you start to need a recalibration and often a leveling kit or lift, and larger tires can rub at full steering lock or under suspension compression. Width matters too: check the rim width range for the new size, and the clearance to the strut and inner fender.

Your speedometer counts wheel revolutions and assumes the original tire size. A taller tire covers more ground per revolution, so you travel faster than the speedometer shows. At +3.5%, an indicated 60 mph is really about 62 mph, and your odometer under-records mileage by the same proportion. A smaller tire does the opposite. The comparison table above shows the true speed at each indicated speed.

Yes. A taller tire effectively raises your final drive ratio, so the engine turns fewer revolutions per mile: slightly calmer highway cruising but weaker acceleration and towing pull. Bigger, heavier tires also add rotating mass and rolling resistance, which usually costs some fuel economy. Large jumps on trucks are often paired with a re-gear for exactly this reason.

Only if the wheel diameter matches (the last number in the size) and the wheel width sits inside the approved rim-width range for the new tire, which the tire maker publishes for every size. Changing the width or aspect ratio on the same wheel is common; changing the wheel diameter means new wheels, and the "plus sizing" approach keeps overall diameter roughly constant by lowering the aspect ratio as the wheel grows.

It works from the nominal dimensions in the size code, which is how every maker specifies tires. A real tire can measure a few tenths of an inch differently by model, tread depth and inflation. It does not check load index, speed rating, offset or brake clearance. Those come from your vehicle placard and the tire maker, and they matter as much as the size itself.

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