Consumer car mag What Car? has launched its True MPG programme, which aims to show the actual, real-world fuel economy of the UK's most popular cars - as opposed to the unrealistic figures claimed by manufacturers.

It's long been accepted that manufacturers' official mpg ratings are more useful as a way of comparing cars like-for-like than an actual indication of day-to-day fuel economy.

However, the first round of True MPG figures released by What Car? (which can be found here) show that the difference between a manufacturer's claimed average figure and the real one can sometimes be a gulf.

To pick on a couple, the Kia Picanto is a massive 26.1mpg less efficient than Kia's claim, while the Audi A6 executive saloon is only 1.5mpg down on Audi's; the A6 is, in fact, 15mpg more economical than the Picanto, which is fairly staggering.

Of course, MPG will never be an exact science, but most drivers struggle to match the official 'average' mpg figure of their car because those figures are achieved in labs using rolling roads - in other words, in conditions nowhere near reality.

What Car?'s test involves each car being driven on a variety of roads, including A-roads, B-roads, motorways and through towns. The test takes driving style, altitude, humidity and temperature variations, and engine temperature changes into account.

Here's a video that explains the test in more detail - skillfully presented by ex-Autoblog writer Tom Webster, as it happens.

Use this tool to find out the True MPG of your car, based on your driving style, mileage, and the conditions you usually drive in.