How do manufacturers calculate mpg?

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Author: | Updated: 14 Oct 2020 11:10

Fuel efficiency. If it’s high on your list of priorities when it comes to choosing your next car, then you’ll be studying its official mpg stats with an eagle eye. However, we know all too well that the official mpg figures manufacturers supply can be a little optimistic, so how exactly are they calculated?

Fuel efficiency – how do manufacturers calculate mpg stats?

Of course, there’s an innumerable number of variables that can affect the amount of fuel you use – from the type of driving you do, your tyre pressures, while even the weather can affect your efficiency. With that in mind, we’ve taken a look how these official figures are collated to see stated by manufacturers are actually an accurate representation…

How is the mpg test done?

New cars are subject to the New European Driving Cycle (NEDC) procedure, which can be used to find out a number of figures which enables manufacturers to find their “official” mpg. The NEDC was created to proved consumers with fair, unbiased data, but recent reports show that few cars can deliver their official fuel consumption figures in the real world.

Official fuel consumption data often differs from the amount of fuel used in the real world?

To understand how the figures are calculated it’s necessary to look at the procedure itself. So the playing field is as level as can be, every test is carried out on a rolling road, with an ambient temperature between 20C and 30C. The engines can’t be run beforehand either. In fact, nothing is left to chance.

The boring bit…

The urban cycle is completed first, and is meant to replicate driving in a built up area. It’s done by accelerating and decelerating from 0mph to speeds of 9, 15 and 20mph. This is repeated three more times and the entire cycle lasts 780 seconds, taking the car 2.8 miles.

The next step is the extra urban cycle, that’s meant to equate to driving on faster roads. Firstly, the car will gently accelerate to 43mph, maintain its speed for 50 seconds and then decelerate to 30mph.

Fuel gauge

It sticks at 30mph for 69 seconds and then accelerates back to 43mph for a further 50 seconds. Finally, it accelerates to 60mph, maintains speed for 35 seconds and then accelerates to 75mph for a further 10 seconds.

The car then slowly decelerates to a standstill and the cycle ends. The figures consumers will be most interested in are not the urban and extra-urban fuel consumption figures, but the combined consumption that is worked out from the two.

Is the NEDC always accurate?

Put simply, no – it isn’t. While the cycle ensures all cars are tested in the same manner, it was last updated in 1997 meaning it doesn’t take into account new hybrid drivetrains, stop-start tech and the power engines have gained over the last two decades.

For example, hybrid models like the popular Mitsubishi Outlander PHEV (a plug-in hybrid) benefit greatly in the test due to its generally slow speed and short distance. The company claims it’s capable of a super-frugal 148mpg, and widely advertise the fact, too.

The Outlander PHEV claims to be capable of 148mpg, but it's not attainable in real world conditions.

In the real world however, these figures are unlikely to be achieved. Aside from its all-electric 32-mile range, the Outlander is fitted with a conventional 2.0-litre petrol engine whereby the average consumption stands at 67mpg. This is calculated using heycars’ “Real MPG calculator” on their Honest John website. 67 mpg is certainly nothing to be sniffed at, but its still less than half its official combined figure.

A fresh approach?

Consumer groups have gone as far as calling the NEDC redundant for the future, not only due to its inaccuracies, but for the fact that the test cycle doesn’t represent real-world driving. After all, there’s no test to simulate driving conditions that make up a “real-life” commute commute in 2016.

Elsewhere, features such as air conditioning are always switched off during testing, which can’t bode well if the aim is to produce a representative and realistic result. But what are manufacturers actually doing to make things less biased?

Following Volkswagen’s dieselgate scandal last year, some are going to greater extents to be honest about their cars’ economy and efficiency. Earlier this year, French firm Peugeot-Citroen started publishing real-world fuel consumption as well as stats from NEDC tests, but we’ll have to see if other manufacturers follow its lead.

PSA Peugeot Citroen recently revealed it will provide a "real world" mpg figure along with official NEDC figures.

It clearly isn’t a perfect test bed for hybrids either – a drivetrain layout more and more manufacturers are starting to adopt – so it seems right that a new test should be used or, at the very least, the current cycle should be overhauled to represent these cars’ economy more accurately.

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