Real MPG: how we test cars

What Car?'s laboratory tests use real-world driving routes to show what fuel economy you can really expect from your next car - here's how we calculate a car's Real MPG...

Real MPG Mercedes GLE emissions test

Fuel economy is an important factor when choosing your next car – after all, you want to make sure you’ll get as many miles as possible from every gallon you put into the tank.

However, the official fuel economy figures published in sales brochures can tempt you into expecting more from a car than it can actually achieve in the real world.

That's why we launched Real MPG, our real-world testing tool which shows you what mpg a car will actually return.

Where do the official figures come from?

When we first introduced Real MPG, official fuel economy figures were based on the New European Driving Cycle (NEDC). This produced figures for urban and extra-urban driving, which were then used to calculate your average fuel economy. However, NEDC results were often unrealistically high, making them pretty much unattainable in the real world.

Since 1 September 2018, all new cars have instead been tested according to the Worldwide Harmonised Light Vehicle Test Procedure (WLTP), which simulates a wider variety of driving conditions than the NEDC, over a longer period of time.

Speeding fines

Our tests have previously found that, while the results generated under the WLTP testing regime are more realistic than those from NEDC testing, they were still almost 5% higher than our real-world figures on average.

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 conditions don't impact the results.

Each test is carried out at Millbrook Proving Ground, which has a testing centre used for conducting regulatory approved vehicle emissions testing. Millbrook has the latest equipment for conducting this testing in a highly scientific manner. 

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 Real MPG data, we show you what you can typically expect from a car if you drive it gently and stick to speed limits, but without resorting to unusual fuel-saving measures or any 'hypermiling' techniques.

Mini petrol station

The Real 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 it's on our rolling road (also 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.

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, the temperature is set 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.

Jaguar I-Pace climate controls.

Headlights are also switched off during testing, while daytime running lights (if fitted) are turned on. Any other electrical equipment – for example a stereo or heated seats – is switched off.

Next, we carry out pre-conditioning tests on the car. This is effectively a dress rehearsal, to check that there are no problems and to make sure that all the cars are in the same state before the real testing begins.

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

After that, it's time for the Real MPG test itself. We sample the car's tailpipe emissions – Carbon Dioxide, Nitrogen Oxides, Carbon Monoxide and Hydrocarbons – on a second by second basis, and collect 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 Real MPG average is calculated, and that's the figure you'll see on our Real MPG tests.

What does the Real MPG result mean to me?

Using our Real MPG results, you can see what sort of fuel economy a 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.

Our data allows you to make an informed decision on your next car – it helps you pick the right vehicle and save money at the pumps.

What about electric cars?

As well as the fuel economy of petrol, diesel and hybrid cars, official battery range figures for electric cars (and plug-in hybrids) are also calculated using the WLTP standard. As with fuel economy, it's often impossible to match a car's WLTP range in the real world.

VW ID Buzz front cornering

One reason for this is because an electric car's range can drop in cold weather, which is why we conduct a winter range test and a summer range test every year. These involve putting several of the latest and most popular models through a ‘drive ’em until they die’ test to see how far they can really go on a charge.

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