Manufacturers’ claimed economy figures have a somewhat mythical air to them. Each model goes through the same official EU cycle test to reach their respective urban, extra urban and combined economy figures, but these tests bear very little resemblance to what’s really achieved in the real world.

So I’ve conducted a bit of an unofficial and impromptu experiment of late in four different cars from the Volkswagen Group (which is at the forefront of downsizing engine technology to help achieve better headline mpg and CO2 figures) to see what car gives you the best chance of matching a claimed economy figure.

First up was my own long-termer, a Seat Leon Ecomotive equipped with a 104bhp 1.6-litre TDI engine. Seat claims 74.3mpg combined is possible; I’m regularly seeing just over 60mpg. Not bad, but some way off the official figure.

Next up was a 104bhp Seat Leon 1.2 TSI. A small capacity petrol engine in a 1280kg family hatchback was always likely to lead to the 52.3mpg figure being someway off. And so it proved; I struggled to reach 35mpg over roughly the same 300-mile test route in a mixture of roads and conditions as all the cars were tested over.

So smaller diesel engines can get close to official figures and small petrol engines nowhere near. What about larger-capacity diesels? I tried the 138bhp 2.0 TDI engine in both a VW Jetta (claimed 58.9mpg) and a VW Passat CC (claimed 60.1mpg).

Both proved to be mightily impressive. The Passat CC returned a highly respectable 57mpg over the test route and the Jetta ended up returning just shy of 70mpg. That’s well in excess of the official figure and this version was not even a full BlueMotion version with aero tweaks, instead being a BlueMotion Technology model with mechanical add-ons including stop-start.

So, back to the science of my particularly unscientific test route and calculations. Bigger seemingly is still better when it comes to engine size. The EU test seems favourable to achieving high economy figures in small cars, yet is as much a judge of larger cars.

Although the figure I got in the Jetta makes a mockery of it. Does anyone have any other interesting economy stories to share?