Everything you need to know about MPG
Miles per gallon - or mpg - is an estimate of
how many miles your vehicle will travel using just one gallon (that’s 4.54
litres) of fuel. In Europe, fuel consumption is usually measured in l/100km
(litres per 100 km).
Mpg has been big news lately, especially since
the Volkswagen emissions scandal broke in 2015. Volkswagen Group installed
software in its cars which ensured the engine performed differently when under
test conditions - like those in which emissions were assessed. This gave a
falsely optimistic view of both fuel consumption and emissions.
Emissions and associated fuel economy have since
been under huge scrutiny and one of the biggest changes has been a new way to
measure consumption which more closely reflects real driving conditions. The
new Worldwide Harmonised Light Vehicle Test Procedure (WLTP) system became
effective in 2017 and has replaced the less accurate New European Driving Cycle
(NEDC) method which started life in the 1980s.
What is WLTP?
The WLTP test cycle was developed using
real-driving data gathered from around the world and therefore better
represents everyday driving profiles. The older NEDC determined values based on
a theoretical driving profile.
The WLTP driving cycle has four parts with
different average speeds: low, medium, high and extra high. Each part contains
a variety of driving phases to simulate real life situations: stops,
acceleration and braking. For each model of car, all engine variants are
tested, along with the heaviest and lightest trim levels.
RDE and diesels
Europe has additional tests for diesel vehicles
which measure pollutants generated during prescribed road tests. These Real
Driving Emissions (RDE) tests don’t replace the WLTP laboratory test, but
complement the results.
There are two phases of RDE. The first phase is
already in place and is used to determine vehicle emissions today. The second
phase - RDE2 - requires a greater level of accuracy. RDE2 applies from January
2020 for new diesel engine types and then from January 2021 for all types.
Why are WLTP and RDE important?
More accurate fuel consumption and emissions
figures under WLTP and RDE mean those quoted for each vehicle will be realistic
- and with careful driving, more likely to be achievable.
WLTP and RDE procedures also provide official CO2 figures, which affects
the amount of tax that new car buyers pay. They also affect company car tax
The new WLTP figures will also help with other
car choice aspects: drivers will be able to base their buying decisions on
likely fuel costs based on their annual mileages and a vehicle’s range on each
tank-full of fuel for example.
Do all new cars show WLTP figures?
Yes. WLTP applied to new car registrations from
September 2018 and stocks of vehicles measured under the old system could be
sold until September 2019.
Does my car have worse consumption under WLTP?
No. Any car with both NEDC and WLTP figures
available will be the same; it’s just the WLTP figures are more realistic.
About the Author
Andrew is a freelance motoring journalist with a background in IT and the vehicle leasing industry. With a lifetime’s passion for all things automotive, he can be found behind the wheel of everything from vans to supercars. In addition to reviewing the latest vehicles and technology, Andrew also runs a couple of classic British motors. He lives at the edge of the Peak District with his son and cat.
Andrew Wright @theMotorWriter