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

BuyaCar team
Dec 6, 2019

If you're looking for a car that's going to save you money, once of the best ways to stem the flow of cash is to look cars with better fuel economy - or the distance a car can travel before it runs out of fuel.

Fuel economy is measured in a few different ways, the most well-known in the UK at least is mpg (miles per gallon), while there is growing popularity for the European version: l/100km (litres per 100 kilometres), getting to know what classes as decent fuel economy can make a big difference to how much you can expect to be spending on fuel over a year.

Back in September 2018, though, the standards by which fuel economy figures are measured were updated. The previous system (NEDC) had become unfit for purpose and, amid growing pressures for reduced reliance on fossil fuels and immediate action to address environmental issues; a tougher and more realistic form of testing (WLTP) was introduced.

The Worldwide harmonised light-duty vehicle test procedure or WLTP was designed to better replicate the kind of driving conditions that are commonplace on actual roads and produce fuel economy figures that were much more representative. For decades, drivers watched in dismay as their cars failed to return anything like their official mpg ratings and cost hundreds of pounds more in petrol and diesel than expected - all because the NEDC tests did not return realistic results.


This means figures published under the WLTP system will be much lower than those published under NEDC. So it may appear that newer cars are actually worse on fuel economy than older models, but this is only as a result of the new test's more stringent and thorough assessment. So it's always good to be sure which test a car has undertaken before putting too much stock into its fuel economy figures.

There's plenty to get into here, so read on for more details on WLTP, what fuel economy actually is, and more.

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% lower than official numbers, with some vehicles as much as 40% 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 do the new WLTP figures affect a car’s fuel economy?

In short, they don’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 have been used to determine the CO2 emissions of all new cars sold since 1 September 2018 but, just to confuse matters, won’t be used for calculating car tax (or company car tax - see below) 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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