In the article below we will look as detailed as possible how car manufacturers calculate official fuel economy figures under Worldwide Harmonised Light Vehicle Test Procedure (WLTP).

What is WLTP?

From September 2018 the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) replaced the New European Driving Cycle (NEDC) test procedure for finding the official fuel consumption figures and CO2 emissions for new cars powered by an Internal Combustion Engine.

While the outdated NEDC test determined test values based on a theoretical driving profile, the WLTP cycle was developed using worlwide real-driving data aimed to represent better everyday driving profiles.

Important to remember that all the manufacturers must adhere to WLTP rules. But there are some low volume makers that can be exempt under Individual Vehicle Approval (IVA) scheme.

What difference does WLTP make?

The new testing regime aims to provide a closer representation of real-world fuel consumption and CO2 figures. Despite all tests will continue to be conducted in a laboratory as it was, there are some differences is the structure of the data resulting from the new test. For petrol and diesel cars, the terms used to describe test cycle driving phases: “urban”, “extra-urban” and “combined”, is no longer be used. Instead, the data is being expressed as "Low", "Medium", "High", "Extra-high" and "Combined".

How the WLTP test is performed?

The test itself is performed indoors in laboratory conditions on a rolling road maintaining an ambient temperature of 23 degrees Celsius. The test takes just 30 minutes, as opposed to the NEDC’s 20 and should start with cold engine. The WLTP driving cycle is divided into four parts with different average speeds: low, medium, high and extra high. Each part contains a diversification of driving stages like stops, acceleration and braking. During the test all the special equipment such as air conditioning or auxiliary heating remain switched off. For a certain car type, each powertrain configuration is tested with WLTP for the vehicle’s most economical version and least economical. Previously, the best figures were used for an entire range.

During the WLTP test, acceleration and deceleration are sharper than during NEDC test, reflecting busier traffic conditions. As a result after all these changes the gap between WLTP fuel economy figures and real world MPG fold below 5%.

WLTP cycle phases in detail

Source: Volkswagen - An overview of WLTP