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Difficulty: HardTyre Safety and Requirements

After driving continuously at high speed on a motorway on a warm day, you check your car's tyre pressures at a service station and find they are 0.3 bar0.3\text{ bar} (4 psi4\text{ psi}) above the vehicle handbook's recommended values. What action should you take?

  1. Leave the tyre pressures as they are and check them again only when the tyres are coldAnswer
  2. B
    Deflate the tyres immediately until the pressure matches the recommended handbook setting
  3. C
    Increase the tyre pressure further to compensate for the additional heat generated by motorway speeds
  4. D
    Replace the tyres as soon as possible because an increased pressure indicates the tread depth has dropped below 1.6 mm1.6\text{ mm}

Answer

Leave the tyre pressures as they are and check them again only when the tyres are cold.
Tyre pressures should always be checked and adjusted when the tyres are cold. Continuous high-speed driving causes friction and internal air heating, which naturally raises the pressure reading. Releasing air from warm tyres means they will become dangerously under-inflated once they cool down.

Step-by-Step Solution

1
Analyze the conditions under which the tyre pressure measurement was taken.
The measurement was taken immediately following high-speed motorway driving in warm ambient conditions, meaning the tyres are hot.
Friction between the road and tyres, combined with continuous flexing, generates significant heat that expands the air inside the tyre.
2
Evaluate the manufacturer's tyre pressure specifications.
Handbook pressure specifications are strictly calibrated for cold tyres.
A pressure rise of 0.2 to 0.4 bar0.2\text{ to }0.4\text{ bar} (3 to 6 psi3\text{ to }6\text{ psi}) is normal and engineered into tyre safety margins when warm.
3
Determine the correct maintenance action.
Do not release air from the tyres; re-check and adjust pressures before driving when the tyres are cold.
Releasing air while warm will cause the tyres to become dangerously under-inflated when they cool down, increasing rolling resistance, fuel consumption, and blowout risk.

Key Concept

Tyre pressures increase naturally when warm due to thermal expansion and must only be checked and adjusted when cold.
Estimated Time:1m 0s
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Topics

Tyre Safety and Requirements

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