HOW THE WHAT CAR? TRUE MPG TEST IS CARRIED OUT

The test is conducted on a rolling road under strictly controlled lab conditions. This means the test is repeatable, because outside variables such as weather and traffic don’t affect the results.

The test is 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.

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.

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.

The climate control is turned on and set to 21deg C. Headlights are switched off during testing. Any other electrical equipment — for example, stereo or heated seats — is also switched off.

Next, we carry out a pre-conditioning test on the car. This is, in effect, a dress rehearsal, 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 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 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.