How 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 conditions don't affect the results.

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

Unlike the official government figures, our tests are based on real-world driving data across a selection of different roads. In other words, although the cars are driven in a laboratory, they’re following a real route we’ve picked out.

Using our True MPG results, you can see what sort of fuel economy your car is really likely to return during real-world use, and our previous results have shown that this is typically different to the results printed in sales brochures - in fact, of the cars we tested in 2016, we found a 24.1% difference between our figures, and the figures from the government tests.

The True MPG test in detail

First, we inspect every car to check its roadworthiness. We then weigh the car, and that weight is used to calculate loads for replicating the resistive forces acting on the vehicle when on our rolling road - commonly known as a 'dyno'. The heavier the car, the more load is applied - this simulates the extra work the engine would need to do to haul around a heavier car on the road.

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 expose any leaks in the system.

If the car has climate control, then we set the temperature to 21 degrees. 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, while daytime running lights (if fitted) are turned on. Any other electrical equipment - for example a stereo or heated seats - are switched off.

Now we carry out pre-conditioning tests on the car. This is effectively a dress rehearsal for the real test, 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 test.

We leave the car to 'soak' at 23 degrees, 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.

Now it's time for the True MPG test. We sample the car's tailpipe emissions - Carbon Dioxide, Nitrogen Oxides, Carbon Monoxide and Hydrocarbons - on a second by second basis, and collect bagged samples of each phase of the test for our fuel consumption calculations.

Our results are summarised by different phases of driving - town, motorway and rural - and fuel economy figures are calculated based on the overall emissions results. Finally, our True MPG average is calculated, and that's the figure you'll see on our True MPG tests.

What will your next car really do to the gallon?

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