Why the EU figures do not represent true MPG

The prescribed EU test - known as NEDC - is a lab test carried out to simulate a mix of different types of driving and arrive at ‘combined’ CO2 emission and fuel consumption figures.

Because vehicle taxation in Europe is now based on CO2 emissions, manufacturers naturally optimise their engines to achieve the lowest possible CO2 in the tests - this skews the actual MPG.

This gives a correspondingly low fuel consumption figure. Unfortunately the relevant EU Directives prescribe that this figure and only this figure can be publicised by manufacturers, even though it is unlikely to be achieved by the average driver in real life conditions.

Imperial urban fuel consumption (mpg) (cold)

The urban test cycle is carried out in a laboratory at an ambient temperature of twenty degrees celsius to thirty degrees celsius on a rolling road from a cold start, ie the engine has not run for several hours. The cycle consists of a series of accelerations, steady speeds, decelerations and idling. Maximum speed is 31 mph (50 km/h), average speed 12 mph (19 km/h) and the distance covered is 2.5 miles (4 km).

Imperial extra-urban fuel consumption (mpg)

The extra-urban cycle is conducted immediately following the urban cycle and consists roughly half steady speed driving and the remainder accelerations, decelerations and some idling. Maximum speed is 75 mph (120 km/h), average speed is 39 mph (63 km/h) and the distance covered is 4.3 miles (7 km).

Imperial combined fuel consumption (mpg)

The combined figure presented is for the urban and the extra-urban cycle together. It is therefore an average of the two other parts of the fuel consumption test, urban and extra-urban cycles, weighted by the distance covered in each part.

What is WLTP?

The new test - called WLTP - aims to test cars under more real-world driving scenarios to achieve more realistic mpg figures.

From September 2017, any new cars launched onto the market will be tested under the new Worldwide Harmonised Light Vehicles Test Procedure (WLTP). Existing models on sale will have until September 2018 before they have to be certified according to the new test.

Although the WLTP will be more reflective of real world driving conditions, it will still be laboratory-based. Meaning that while it will create a more accurate driving profile, it can't account for variations in weather, car occupancy and traffic congestion - all of which have a big effect on fuel economy.

In June 2018, the Government proposed that all manufacturers change over to the new WLTP fuel consumption figures in their promotional material and advertising for all vehicles on the same date - 1 January 2019. This aims to save the confusion of having NEDC and WLTP figures on brochures.

It was also proposed that the change-over to WLTP specific CO2 emissions should take place from 6 April 2020. This will align with the use of the new CO2 figures for VED and company car taxation purposes.

What's changing?

The WLTP will be longer, changing from 20 minutes to 30 minutes and the test will start at a temperature of 14°C rather than the current 20°C to 30°C, a better representation of the UK climate.

It will also cover more than twice the distance - 14 miles at a higher average speed - and include more braking and accelerating to reflect situations that drivers experience in everyday life. Extra weight and power used by optional equipment will be taken into account as well.

The new WLTP aims to correct many of the issues with the current NEDC. It will no doubt yield figures that reflect more realistic fuel consumption figures, which means that the advertised MPG figure is likely to go down.

How will this affect CO2-based tax?

Currently, each car receives a certificate of conformity which shows its CO2 emissions level based on the NEDC. During the transition from NEDC to WLTP, cars approved before then will continue to have CO2 values as measured under the current NEDC test only.

When a new car is certified after September 2017, its official documents will have CO2 emission values from the new WLTP as well as the NEDC.

The change in CO2 would in theory affect the first year 'showroom tax' rate of a new car. This means that a vehicle currently on sale could be subject to more first year tax if the new WLTP test shows an in increase in CO2, which is very likely. 

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