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Difficulty: Very hardTyre Safety and Requirements

You are preparing your car for a long motorway journey carrying a full complement of passengers and heavy luggage. A space-saver spare wheel is currently fitted to the rear of the vehicle. Upon inspection, you find that your standard tyres have a tread depth of exactly 1.6mm across the central three-quarters of their breadth. Which of the following is the correct procedure to ensure your vehicle is legally compliant and safe for this journey?

  1. A
    Maintain the standard normal-load tyre pressures to prevent over-inflation risks under the extra weight, and restrict your maximum speed to 50 mph (80 km/h).
  2. B
    Replace the standard tyres before the journey, as a 1.6mm tread depth falls below the legal minimum requirement when a vehicle is fully loaded.
  3. Increase the pressure of the standard tyres to the manufacturer's heavy load specification when they are cold, and restrict your maximum speed to 50 mph (80 km/h).Answer
  4. D
    Adjust the tyre pressures to the heavy load specification while the tyres are hot, and proceed at a reduced motorway speed of 60 mph (96 km/h).

Answer

Increase the pressure of the standard tyres to the heavy load specification when cold, and do not exceed 50 mph.
To ensure safety and legal compliance on a journey, tyre pressures must be increased to accommodate heavy loads, and this adjustment must be made when the tyres are cold to obtain an accurate reading. Furthermore, vehicles fitted with a space-saver spare wheel are legally restricted to an absolute maximum speed of 50 mph (80 km/h). The 1.6mm tread depth, while at the absolute minimum limit, is legally sufficient regardless of the heavy load.

Step-by-Step Solution

1
Identify the speed restrictions associated with a space-saver spare wheel.
The maximum permitted speed is 50 mph (80 km/h).
Space-saver tyres are smaller and offer less grip than standard tyres, making them unsafe at high speeds, which strictly legally limits them to 50 mph.
2
Evaluate the tread depth requirement against the scenario parameters.
The tread depth of 1.6mm across the central three-quarters is legally compliant.
The UK statutory minimum tread depth requirement for cars is exactly 1.6mm, and this legal threshold applies regardless of how heavily the vehicle is loaded.
3
Determine the correct procedure for managing tyre pressures when carrying a heavy load.
Tyre pressures must be increased to the manufacturer's specific heavy load recommendations.
Extra weight compresses the tyres significantly; increasing the pressure prevents dangerous overheating, reduces the risk of blowouts, and ensures the vehicle remains stable to handle.
4
Identify the correct environmental condition for checking and adjusting tyre pressures.
Tyre pressures must always be checked and adjusted when the tyres are completely cold.
Driving generates heat which causes the air inside the tyre to expand, giving a falsely high pressure reading. Adjusting hot tyres will result in dangerous under-inflation once they cool down.

Key Concept

Synthesizing legal requirements for minimum tread depth, load-dependent tyre pressure adjustments, and speed restrictions for temporary space-saver wheels.
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Topics

Tyre Safety and Requirements

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