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Glossary Science & Engineering

Tire size code

A tire size such as 225/45R17 gives the width in millimetres (225), the sidewall height as a percentage of width (45) and the rim diameter in inches (17).

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In more detail

Overall diameter is rim + 2 × width × aspect / 25.4: 24.97 in for 225/45R17. Switching to a 245/40R18 (25.72 in) makes the speedometer read about 3% low.

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See also

  • Glossary Fuel economy
    Fuel economy is how far a vehicle travels on a unit of fuel (miles per gallon.
  • Cheat sheet Vehicle Formulas Cheat Sheet
    Fuel cost, mpg and L/100 km, tire diameter and speedometer error, horsepower from torque or trap speed and EV charging cost.
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