How We Calculate True MPG

Fuel economy is an important factor when choosing a new car; after all, you want to make sure you’ll get the best mileage possible for every gallon you put into the tank.

However, the official fuel economy figures published in sales brochures can lead you to expect unrealistic returns. That’s because those figures are based on something called the New European Driving Cycle (NEDC) – a government test that’s wholly unrepresentative of how we actually drive our cars on the road.

That's why we created True MPG, a fuel economy test that’s based on real-world driving data to make it much more realistic than the official tests but is conducted in a laboratory for the purposes of repeatability.

What does the True MPG result mean to me?

Our True MPG results give you a good indication of the fuel economy you can expect from a car on a variety of different types of road. Typically, the results are very different from the figures printed in sales brochures; in fact, based on cars we tested in 2016, True MPG results are 24.1% lower, on average, than the official figures.

Some cars can be as much as 40% less efficient in the real world than they are in the official government tests, while, confusingly, others can actually be slightly more frugal in real-world driving.

Our True MPG results show you what you can typically expect from a car that's driven gently and within speed limits, but without resorting to unusual fuel-saving measures or any 'hypermiling' techniques.

So, how do we do it?

Our tests are conducted on a rolling road under strictly controlled laboratory conditions. This means our tests are repeatable, because outside variables such as weather and traffic don't affect the results.

Tests are carried out at Millbrook Proving Ground, a centre used for regulatory approved vehicle emissions testing. Millbrook has the latest equipment for testing vehicles in a highly scientific and repeatable manner.

The True MPG test in detail

First, we inspect the car to check its roadworthiness. We then weigh the car, and that weight is used to calculate the rolling road (or dyno) loads that replicate the resistive forces acting on a car when it’s being driven on the road. The heavier the car, the greater the load that’s applied; this simulates the extra work the engine would need to do to haul around a heavier car.

Next, the tyre pressures are checked and an exhaust connection is fitted, allowing the car's emissions to be measured. We also carry out an exhaust pressure check to identify any leaks in the system.

If the car is fitted with climate control, we switch this on and set the temperature to 21deg C. If the car has manual air conditioning, the temperature is set to its midway point and the fan to its slowest speed.

Headlights are switched off during testing. Any other electrical equipment – for example, stereo or heated seats – are also switched off.

Next, we carry out a pre-conditioning test on the car. This is, in effect, a dress rehearsal for the real thing, to check that there are no problems and to make sure that all the cars we test are in the same state before they start the True MPG test.

We leave the car to 'soak' at 23deg C, usually overnight, so that every car starts the test with the same engine temperature. We don't place the car's battery on charge while this is going on.

Then it's time for the True MPG test. We sample the car's tailpipe emissions – carbon dioxide (CO2), nitrogen oxides (NOx), carbon monoxide (CO) and unburnt hydrocarbons (HC) – on a second-by-second basis and collect bagged samples of each phase of the test for our calculations.

Our results are summarised by different phases of driving – town, motorway and rural – and fuel economy figures are calculated from CO2 results. Finally, our headline True MPG average is calculated.

Pre-2016 testing

Our pre-2016 test results were obtained using our old testing procedure, in partnership with Emissions Analytics. Before 2016, we assessed real-world fuel economy on the road using a portable emissions measuring system (PEMS), rather than in the strictly controlled conditions of a laboratory.

These older figures give you a good guide to the fuel economy you can expect from a car, but they can’t be directly compared with newer laboratory results.

What will your next car really do to the gallon?

Find your True MPG

Fuel tips to improve fuel efficiency

You don’t need to be a professional driver to be more fuel efficient. Follow these 10 handy tips to improve efficiency and get the best mpg from your car.

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