Mpg meaning & WLTP: can you trust the new fuel economy test?

What is mpg? Everything you need to know about fuel economy, including the new WLTP test

Craig Thomas
Jul 31, 2018

Fuel economy figures are changing - at last. For decades, drivers have watched in dismay as their cars fail to return anything like their official mpg ratings, costing hundreds of pounds more in petrol and diesel than expected.

But from September, every new car must go through a new and more realistic fuel economy test, called WLTP (Worldwide harmonized light-duty vehicle test procedure). It’s claimed to make mpg figures more accurate, so drivers can better estimate fuel costs.

It’s likely to be confusing at first because the new mpg figures are likely to be worse, making it appear as if cars have become more inefficient. Manufacturers will also publish the new figures alongside the old ones in brochures, so many buyers won’t know what to believe.

However, the change could be good news if you’re looking for a car. It’s expected to lead to bigger discounts on new and nearly new models that haven’t passed the new test. These can’t be sold as new from September.

In 2020, the new test will have an impact on car tax and company car tax.

Scroll down for more information on what mpg means or jump to new WLTP test details.

What is mpg?

Mpg stands for miles per gallon and is a measure of a car’s fuel economy. It’s an estimate of how far a vehicle can travel on a gallon of fuel, based on a standard laboratory test, but doesn’t fully replicate the varied conditions of real-world driving.

The standard test, which every type of new car must go through, is needed to ensure that mpg figures are comparable from one vehicle to another but it usually results in a sizeable difference between the official mpg figure and the less-impressive fuel economy that you can expect in the real world.
How can I get a better idea of what my car’s true economy figure should be?
A number of independent companies that have conducted extensive tests that offer more accurate mpg figures than the official numbers.

One British company, Emissions Analytics, has tested over 800 vehicles over the last five years, covering a wide range of makes, models and engine types. Its Equa Index also enables the company to forecast fuel economy for cars it hasn’t yet tested.

The company’s tests have found that the average car’s fuel economy is 25 per cent lower than official numbers, with some vehicles as much as 40 per cent below the laboratory figure.

This year’s changes to the way that fuel economy is calculated should decrease the discrepancies but Emissions Analytics says that they will still be subject to many of the same issues as precious tests because they will still be carried out in a laboratory.
What is WLTP?

Until recently, every car’s official fuel economy figure was calculated in a laboratory test procedure called the New European Driving Cycle (NEDC).

Cars were placed on a rolling road and went through exactly the same process, involving accelerating, maintaining a steady speed, braking, and stopping over 20 minutes.

The standardised test originated in the 1970s and was designed for a different era of cars without the power or equipment of modern vehicles. There were also several loopholes that manufacturers could take advantage of to legally boost their cars’ mpg figures - by over-inflating tyres, for example.

It meant that drivers were increasingly complaining of huge disparities between the official figures and those that could be attained in real-world driving. Independent analysis such as the Equa Index showed just how misleading the official figures could be.

Plans for a new testing regime were already in hand before the Volkswagen’s emissions cheating hit the news in 2015, but that scandal increased the focus on air quality and the need for a rigorous new test.

So the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) was introduced in September 2017 and as of 1 September 2018, all new car models have to be tested according this new process.
How does the WLTP test work?

The WLTP cycle has been developed using real-driving data, gathered from around the world, so it should be a better representation of how people really drive than the old NEDC test.

It will still be carried out in a laboratory to ensure standard conditions for every car but the differences include:

A four part test with different average speeds: low, medium high and extra high. These represent urban, suburban, main road and motorway conditions.

More realistic ambient temperature that’s common in Europe

More acceleration and braking, with shorter stops

Testing cars with different equipment levels to show how optional extras can affect mpg.

With all these changes, the WLTP test should provide a much more accurate basis for calculating a car’s fuel consumption and emissions, although there’s still likely to be a discrepancy between the official figures and real-world fuel economy.
What’s the connection between mpg and CO2?

Fuel economy figures and carbon dioxide emissions (expressed in g/km) are directly related. For example, a diesel car emitting 95g/km of CO2 has fuel consumption figure of around 76.3mpg, while a petrol car consumes 70.6mpg for the same CO2 emissions.

As well as mpg, the NEDC and WLTP procedures provide official CO2 figures, which affects the amount of tax that new car buyers pay, as well as company car tax rates.

Until 2020, the results produced in the WLTP test will be be converted to the old, less accurate NEDC standard, for tax purposes. But from 2020, the WLTP figures will be used to calculate tax.

As the test is tougher, official CO2 emissions are expected to rise, which will have the most impact on business users. If you’re buying a car that has been tested under the WLTP procedure, it’s worth checking that you won’t face a sharp increase in tax from 2020.
How will the new WLTP figures affect a car’s fuel economy?
In short, they won’t.

The WLTP figures are the results of official laboratory tests, so they don’t affect the figures that owners get from their car when they drive it.

There are a huge number of factors that affect what fuel consumption a car returns – a figure that can often be found in the trip computer of a car. For example, if a driver accelerates harshly or brakes late and more suddenly, their fuel consumption will increase. Conversely, if a driver does everything more smoothly, anticipating any required changes in acceleration or braking, they will be better at optimising their fuel consumption.

Equally, if a car is mostly used in stop-start urban traffic, fuel economy won’t be the best, while a car that is used more on longer motorway runs, at a more constant speed, is operating more efficiently.

There are also numerous other tips and tricks that can be utilised to maximise fuel economy (try and drive with an empty boot as much possible – so only keep things such as sets of golf clubs or other sporting equipment in the boot when actually using them) and all these factors can affect a driver’s average fuel consumption. Indeed, if a driver can return a figure that approaches the official manufacturer’s figure, they should congratulate themselves on their economical driving style.

The new WLTP figures should be closer to real-world figures than the old NEDC figures, but they’re arrived at after testing in a lab, so there will still be a difference between them and what drivers actually are able to record.
What effect will WLTP have on hybrid and electric vehicle figures?
Plug-in hybrid vehicles (PHEVs) have returned highly impressive mpg and CO2 figures under the previous NEDC testing regime – but buyers have been less impressed when they’ve realised what the real-world figures are.

This is because the test was highly complex – and very forgiving. The International Commission for Clean Transportation (ICCT) described the NEDC test as “not a demanding test cycle and hence not representative of real-world driving, [resulting] in rather optimistic estimates of energy consumption as well as PHEV electric range.”

Even though the ICCT welcomes the new WLTP test, it said: “Nevertheless, even under WLTP, official and real-world values for PHEV will continue to strongly deviate for some customers.”

It’s a similar story with the officially quoted driving ranges of electric vehicles (EVs), which are also determined using the same testing procedure.

This means that PHEVs and EVs are subject to the same caveats as petrol and diesel cars, in terms of their real-world performance, with buyers needing to be aware that the same efficient driving strategies will be required to return figures that are in any way close to the official figures.
How will WLTP affect car tax?
The new WLTP figures will determine the CO2 emissions of all new cars sold after 1 September 2018 but, just to confuse matters, won’t be used for calculating car tax – officially called vehicle excise duty (VED) – until April 2020.

This means that between September 2018 and April 2020, the government will continue to use NEDC figures, but they will be what’s known as NEDC correlated figures, which are calculated by converting WLTP figures. As this is only an estimate, the NEDC correlated figures can be different to those for a similar vehicle tested under the old NEDC regime.

This means that the manufacturers will quote WLTP and NEDC figures in their publicity. If buyers want an indication of how much car tax they’ll pay, until April 2020, they should use the NEDC figures as a guide.
How will WLTP affect my company car tax?
As the benefit-in-kind tax that company car drivers pay is also based on CO2 figures, the same rules apply as regular car tax, in that WLTP figures will only be used from April 2020, with NEDC-correlated figures used until then.

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