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Car fuel economy

Official fuel consumption figures

Car manufacturers have to publish them but you can’t rely on the numbers

The gap between the fuel economy you get in the real world and the official figures that car manufacturers must quote in their brochures has been getting wider and wider.

Back in 2001 the average gap was a relatively small 8% but by 2014 this had grown to 40%.

Some difference is to be expected as a laboratory test can never accurately reflect the range of driving style, road and weather conditions in the real world, but the gap is now so wide that manufacturers have been accused of misleading claims.

It seems that the more economical a car is to run, on paper, the greater the gap between official figures and actual fuel consumption.  This means you can’t even rely on official figures as a guide to relative performance.

Fuel consumption

Official tests

New car models have to meet a range of safety and environmental ‘type approval’ tests before they can be sold. 

  • One such test is to determine official fuel consumption and CO2 emissions data.
  • The car’s ‘driven’ on a rolling road, following a defined cycle.
  • Car manufacturers have to state official fuel consumption figures and aren’t allowed to quote alternative figures.

The official CO2 figure is the basis for our car tax system and is used by the European Commission to monitor fleet-average CO2 emissions and progress towards emissions reduction targets.

CO2 reduction targets

European countries must meet strict targets for fleet average CO2 emissions – 130g/km from 2015 and 95g/km from 2020.  Heavier cars are allowed higher emissions if offset by lighter vehicles with lower emissions.

Car manufacturers have a strong financial interest as the fines for exceeding CO2 targets can be substantial.

Laboratory test cycles

The official test takes place on a laboratory rolling road and currently follows something called the New European Driving Cycle (NEDC) which has two parts: an ‘urban’ cycle and an ‘extra-urban’ cycle.

  • Urban – a cold start followed by around 2.5 miles at an average of 12mph and briefly reaching a maximum of 31mph.  
  • Extra-urban – following straight on from the urban cycle, the car covers 4.3 miles at an average of 39mph.
  • Three official fuel consumption figures are calculated: urban, extra-urban and a ‘combined’ figure which is a weighted average of the other two.
Issues with the current test

The basic test cycle was developed in the 1970s and doesn’t properly represent modern driving patterns or vehicle performance.

  • The cycle’s only short in duration and dominated by periods of idling and low engine load.
  • The test car only briefly reaches motorway speed.
  • Air-conditioning, lights and other electrical loads are all switched off for the test.
  • Flexibilities in the test procedure can be exploited to achieve a lower overall fuel consumption
  • The test is run in a room temperature between 20 and 30 C – much higher than the average outside temperature.
  • The way that the rolling resistance of the rollers is set in discreet steps means that a small reduction in vehicle weight may be enough to get a more significant reduction in rolling resistance.
  • Fuel consumption can be optimised for the test conditions.
  • The test can give a distorted view of the benefits of new technologies – stop/start systems work well in the test because it contains a lot of idling: about 10% of the NEDC.
  • The test doesn’t take account of passengers or other loads

A new cycle and test procedure

A new global driving cycle, the World Light Duty Test Cycle (WLTC) has been developed by experts from Europe, India and Japan with the USA and China as observers.

In Europe, new test procedures based on the WLTC should be in use from 2017.

The change from NEDC to WLTC will result in an increase in fuel consumption and CO2 emissions of around 5%.  Changes to the test procedure, vehicle set-up and operating conditions will have an affect too.

The Manufacturers' fleet average CO2 emissions target of 95g/km from 2020 will have to be changed as this was set under the NEDC regime.

Toxic emissions in ‘real driving’

European regulations require toxic exhaust emissions to be effectively limited in normal driving and throughout the vehicle’s normal life rather than just in standard laboratory tests.

To achieve this a new 'Real Driving Emissions' (RDE) test is being introduced alongside the new laboratory test.

RDE uses a Portable Emissions Measurement System (PEMS) that measures NOx, CO and soot (particulates) on a vehicle while it’s driven on normal roads in a range of conditions.

The new RDE test will apply to all new models from September 2017 and to all new vehicles from September 2019.

RDE is a relatively severe test and higher emissions limits will apply compared to the laboratory WLTP test. A so called 'conformity factor' will be applied in two steps:

  • Step 1: RDE emissions can be up to 2.1 times (110%) higher from September 2017
  • Step 2: RDE emissions can be up to 1.5 times (50%) higher from January 2020 (new models) and January 2021 all new cars.

Buying a new car

If you assume fuel economy around 40% less (worse) than the official combined figure quoted by the manufacturer you shouldn’t be disappointed, and may be pleasantly surprised.

If you're looking at a used car the gap will depend on the age of the vehicle – around 10% for a 10-year old car up to around 25% for a car that's 3 to 5 years old.

Any single test is flawed and no single driver is representative of everyone else, but if you could see what sort of fuel economy lots of other drivers are getting in the same car that would give you a fair idea of what can be achieved taking your own driving style into account.

  • Real MPG (Honest John) – users have uploaded more than 100,000 fuel consumption figures.  On average cars achieve 84% of the official combined fuel consumption figure but this includes many older models when the test was more representative.
  • Spritmonitor.de – a German site which calculates fuel economy based on users’ fuel purchase and mileage data. Fuel consumption figures are quoted in litres/100km rather than miles per gallon. (To convert litres/100km to mpg divide 62.15 by the fuel consumption in l/100km and multiply the result by 4.546)

1 February 2017