The biggest difference between our True MPG fuel economy tests and the official tests is that ours are conducted in real-world conditions rather than in a laboratory. Our test route starts with urban driving at an average speed of 15mph. This is followed by extra-urban roads (dual carriageway and motorway) at an average of 60mph, before the test concludes with another urban loop.
Engines are less efficient when cold, so we always make sure that cars are fully up to temperature before starting a fuel test. Then, once it has begun, our testers drive at a steady pace, avoiding heavy acceleration and braking whenever possible.
Two-seater cars are tested with one person onboard (the testing equipment has to go on the passenger’s seat) and other cars with two people. At the end of the test, the average fuel consumption on the test route is calculated from the CO2 emissions. However, the volume of traffic inevitably varies, so this economy figure is just a starting point.
Sensors attached to the car’s electronic brain record the road speed, exhaust manifold pressure and throttle position throughout the test, while other equipment measures the altitude, humidity and air pressure at each given moment. These variables all have to be factored in before we know a car’s True MPG.