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Why must gear-shifting points for manual transmissions during a WLTP test be calculated based on engine power availability rather than using fixed speed-based shift markers?

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The WLTP, or Worldwide Harmonized Light Vehicles Test Procedure, is a global standard used to measure fuel consumption and exhaust emissions. It uses a dynamic test cycle that mimics real-world driving. Gear-shifting points must be calculated based on engine power availability, rather than fixed speeds, because vehicles vary significantly in their mechanical design, weight, and engine performance. A fixed speed marker, such as shifting into fourth gear at 50 kilometers per hour, would be inappropriate for both a high-performance sports car with a short-ratio gearbox and a heavy-duty commercial van with a long-ratio gearbox. Calculating shift points based on power availability ensures that the engine operates within its optimal efficiency range for every specific vehicle type. This process involves defining a normalized power index, which compares the actual power of a vehicle to its curb weight and the power required to complete the test cycle. By using this index, the test accounts for the available power-to-mass ratio of the vehicle. If shift points were fixed, a less powerful vehicle might be forced to shift into a high gear where it lacks the torque to maintain speed, causing it to labor and produce unrepresentative emissions. Conversely, a powerful vehicle might stay in a low gear unnecessarily, leading to excessively high engine speeds and inaccurate fuel consumption data. Using power-based calculations ensures that every vehicle undergoes the test under load conditions that reflect its unique engineering, making the test fair, repeatable, and representative of how that specific vehicle performs on the road.

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Why must gear-shifting points for manual transmissions during a WLTP test be calculated based on engine power availability rather than using fixed speed-based shift markers?

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