Real MPG

real MPG urban extra urban MPG

Every new car on sale in the UK undergoes a standard official test which is conducted on a ‘rolling road’ used to simulate an imagined journey. The set of driving speeds and time periods used is known as the New European Driving Cycle (NEDC).

The test is intended to determine a particular model’s fuel consumption, CO2 emissions as well as 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 NEDC is composed of two parts: an ‘urban’ cycle to reflect driving in towns and cities; and an ‘extra-urban’ cycle to reflect driving on faster roads. These two test results are then combined to provide the official ‘combined’ fuel economy and CO2 emissions.

The urban cycle starts with a cold vehicle and covers a stop start journey of 2.5 miles at an average speed of 12 mph. The test vehicle briefly reaches a maximum speed of 31 mph. Following the urban cycle, the extra-urban cycle starts with a warm vehicle and covers a distance of 4.3 miles at an average speed of 39 mph. The journey mixes acceleration, deceleration, steady speed and idling.

However, it is widely acknowledged that the official NEDC cycle is not fully representative of real-life driving conditions. Indeed, there is much evidence that it is far from reality and is increasingly diverging from real world performance.

What is Real MPG?

‘Real MPG’ figures as published on Next Green Car are intended to provide a realistic indication of a vehicle’s fuel economy as experienced in real world conditions. While driving conditions vary widely, Real MPG attempts to more closely represent normal driving than does the NEDC cycle.

If the Next Green Car post box is anything to go by, the difference between ‘official MPG’ and ‘Real MPG’ is a frustration shared by almost all UK motorists. What is more, there is a growing body of evidence that the discrepancy is increasing year-on-year.

Research conducted by the International Council for Clean Transportation (ICCT) in 2013, for example, analysed several large sets of on-road driving data from various European countries. The research found that the difference between the official test figures and real-world driving increased from less than 10% in 2001 to 25% in 2011.

Another study conducted by Transport & Environment (T&E) in 2014, found that the gap between real-world fuel consumption and carmakers’ claims has widened from 8% in 2001 to a staggering 31% in 2013 for private motorists and 45% for company cars.

According to recent estimates from Emissions Analytics, not only do petrol engines have worse official fuel economy than diesels, but the gap compared to real-world fuel use is larger. Their analysis also showed that while manuals return a better fuel economy than automatics (in general), automatics have a smaller discrepancy between official and real-world figures.

To account for the discrepancy between official and real-world figures, Next Green Car use a set of correction factors based on fuel type and year to estimate Real MPG for all models included in the NGC database. This includes over 50,000 new and used models launched since 2001.

Download the T&E report: Mind the Gap (PDF)

Why are the official figures wrong?

As highlighted in the Transport & Environment (T&E) report, car manufacturers have a vested interest in optimising their vehicles to achieve the highest fuel economy and lowest possible CO2 emissions. Under current EU legislation, each manufacturer’s average fleet emissions must meet a target of 130 g/km by 2015 and 95g/km in 2020. Should these target not be met, the companies involved will face substantial fines.

Official CO2 figures are also used in many EU Member States to determine levels of vehicle taxation. In most cases, a low CO2 figure means low annual vehicle tax; making low emission models more attractive to car buyers. A company’s vehicle sales, therefore, are at stake.

The T&E report provides details of how car makers manipulate the official MPG and CO2 figures. In the first half of the test, during which the air and rolling resistance of the car is measured, it is now commonplace for carmakers to adjust the brakes, pump up the tyres, and tape up all the cracks around the doors and windows to reduce the air and rolling resistance.

While there is no evidence that carmakers are breaking any formal rules, the rules themselves are insufficient to get meaningful results and the test procedures so lax that there is ample opportunity to massage the test results.

Other common practices, for example, include disconnecting the battery during the test, minimizing the weight of the car, using special lubricants that are not supplied with the production vehicle and testing in unrealistically hot temperatures. The current procedure also inexplicably allows the CO2 results declared by the manufacturer to be up to 4% below the measured results.

Independent tests conducted using the official drive cycle but with regular production vehicles and without using all the loopholes in the rules, produce results for CO2 emissions and fuel consumption which are 19%-28% (average 23%) higher than the official figures reported. About half of this is explained by differences accruing from the road load testing, the other half comes from differences in the laboratory testing.

Forthcoming Worldwide Light Vehicles Test Procedure

Given the issues highlighted above, a new global testing system, named the Worldwide Harmonized Light Vehicles Test Procedure (WLTP) has been developed, led by the United Nations Economic Commission for Europe (UNECE). According to most analysts, the new cycle, which will be finalised in Spring 2015, is more representative of real-world driving and the test procedures are more robust as compared to the existing NEDC.

The WLTP consists of three different cycles, depending on vehicle class defined by power-weight ratio PWr in kW/Tonne (rated engine power / kerb weight). In each class, there are several driving tests designed to represent real world vehicle operation on different types of road. The change in driving cycle from (NEDC to WLTP) according to some estimates will result in an increase in fuel consumption/CO2 emissions of around 5%.

The European Parliament and European Commission have proposed that the WLTP be introduced in 2017. However, many carmakers are opposing the introduction of the new test and are trying to delay its implementation as it is likely to reduce their ability to manipulate test results in the future.

During 2015, Next Green Car will be expanding its coverage of the WLTP and assessing ts implications for future fuel economy data.

Ben Lane

Author:Ben Lane
Date Updated:10th Mar 2015

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