Real MPG

real MPG urban extra urban MPG

Every new car on sale in the UK undergoes a standard official test which is conducted in a laboratory, on a 'rolling road' used to simulate an imagined journey.

The test is intended to determine a particular model's fuel consumption, as well as its CO2 emissions and four other so-called ‘regulated’ pollutants. These values are then used to provide the 'official' fuel economy and CO2 emissions, the latter determining how the vehicle is taxed for use on UK roads.

The current set of tests is known as the Worldwide Harmonised Light Vehicles Test Procedure (WLTP). This is a recent implementation though, and the legacy New European Driving Cycle (NEDC) is still being used during the switchover, to keep comparisons between new and older cars possible for customers.

Both NEDC, and to a lesser extent, WLTP have been criticised as providing results that don't reflect what is possible under real-world driving conditions. Drivers are rarely able to achieve quoted official fuel economy figures, and the increased amount of fuel used also means official emissions statistics are innacurate too.

Find out more about Emissions on NGC's microsite

Real MPG

To counter the issues thrown up by the official WLTP and NEDC tests, Next Green Car publishes 'Real MPG' figures alongside each of the more than 60,000 models in our database. Real MPG is intended to provide a realistic indication of a vehicle's fuel economy as experienced in real-world driving conditions. They are figures that should be achievable by buyers - even taking into account variable driving conditions - and aim to more closely represent normal driving than the lab-based test cycles.

Next Green Car's information uses data provided by Emissions Analytics for as accurate results as possible. The independent vehicle testing company uses Portable Emissions Measurements Systems (PEMS), fitted to cars that are then driven on public roads. This provides data that is far more accurate than that calculated in a laboratory, improving the efficacy of its fuel economy, air quality, and emissions ratings.

Worldwide Harmonised Light Vehicles Test Procedure

The current WLTP has been developed to counter widely accepted failings in the suitability for purpose of the NEDC. Few drivers can achieve their car's 'official' fuel economy figures from the NEDC tests, so the WLTP has been established as a replacement protocol, and one that can be implemented around the world. Essentially it is more representative of real-world driving thaan NEDC, and the test procedures are more robust too.

The current WLTP is made up of four driving phases broken into two main parts - urban to simulate town driving, and extra urban for open road driving. These phases are carried out over the course of 30 minutes. A maximum speed of 81mph is reached over 14 miles, at an average speed of 29mph. Gear shift points vary for each vehicle, and features such as air conditioning are taken into account for the final emissions and fuel economy reatings too.

The WLTP represents an improvement over the NEDC test which was shorter in both time and distance, while also featuring slower average and maximum speeds. It also had fixed gear change points and didn't take the influence of optional equipment into account, and fetured two phases.

Laboratory Testing

Graphic courtesy of European Automobile Association (ACEA)

Like NEDC tests, the WLTP is conducted under laboratory conditions, with driving phases simulated rather than carried out on actual roads. As such, there will be elements that are unable to be calculated accurately, and the problem of inaccurate fuel economy and emissions figures remains - even if they will be better than under NEDC. However, the benefit between lab-based and real-world tests is that the former can more easily be recreated, allowing for greater accuracy when comparing one model to another.

New European Drive Cycle

Despite being phased out, the NEDC remains an important part of vehicle testing during the crossover to WLTP figures. NEDC figures are still quoted currently as buyers require a basis from which to compare vehicles by.

WLTP & NEDC tests

Graphic courtesy of European Automobile Association (ACEA)

The test is fundamentally flawed though, with inaccurate figures produced from NEDC results, with the gap between real-world and official statistics increasing rather than shrinking over time - as highlighted in the T&E Mind the Gap report. This study by independant environmental transport group Transport & Environment shows that the average gap between real-world economy figures and those quoted from the NEDC tests has risen from 9% in 2001 to 42% in 2016. You can read the report via the link below.

T&E: Mind the Gap

Real Driving Emissions

Real Driving Emissions (RDE) is the newest test protocol to be adopted by European countries, and works on the same testing principles as those carried out by Emissions Analytics amongst others.

The use of PEMS technology allows testers to gather the most accurate amount of information possible, collected as a vehicle is driven around a set route on public roads. RDE testing has started for new car types, and will apply to all types of car sold from September 2019.

Real Driving Emissions test

Graphic courtesy of European Automobile Association (ACEA)

Because of these more accurate results achieved though, it is expected that many cars will be deemed as illegal by failing to meet emissions criteria laid out in current Euro Emission Standards. To make sure that these cars aren't outlawed overnight, there will be a phased implementation of conformity factors for NOx emissions. This begins with a factor of 2.1 from September 2017, before phase two begins in January 2020 with a confority factor of 1.0, plus an error margin of 0.5.

Essentially this means that car's will be 'allowed' to emit more than twice the legal limit for NOx emissions until 2020 - though in reality emissions won't have gone up in particular, it's just that the tests are more accurate.

RDE is not intended to replace WLTP but to supplement it. By using both protocols, in a few years' time, there should be data available for buyers to both compare vehicles side-by-side thanks to WLTP results, and see what the car is able to do in real-world driving conditions because of RDE results.

The PSA Group - which comprises Peugeot, Citroen, DS Automobiles, and Opel/Vaxhall - should be commended for taking a proactive approach to RDE testing, implementing its own programme of tests independently verified by external auditors. The results include most of the Peugeot, Citroen, and DS Automobiles models currently on sale. Opel/Vauxhall figures are not currently available because the PSA Group purchased the brands after testing began. However, since the PSA Group's tests are on-going, it is expected that Opel/Vauxhall models will be added to the data sets, aolong with new models from Peugeot, Citroen, and DS Automobiles.

Testing by the PSA Group has been designed to reflect the official RDE test processes, and covers 14 miles on urban roads, 25 mies on open roads, and 17 miles on motorways. The tests are conducted under real driving conditions, including the use of air conditioning, luggage carried, passengers, hilly terrain etc.

Chris Lilly

Author:Chris Lilly
Date Updated:12th Dec 2017

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