Some of you will have read our Fiat 500 TwinAir road test and noted that our average economy figure of 35.7mpg comes in a touch short of the claimed numbers. Or rather it's almost half the combined figure of 68.9mpg that Fiat reckons its blown two-cylinder motor should achieve in the 500. First of all, it's worth addressing the question of how we get our economy figures. Our average figure is taken with a brim-to-brim measurement recorded over a distance of more than 300 miles including motorway, b-road and town driving. It's representative of an ordinary and varied route driven at normal road speeds, and gives a realistic idea of what economy you can expect to return under similarly varied conditions. Our track economy figure is taken separately and not included in the average economy figure.

 Our touring figure is taken over a course at Mira's proving ground that is easily repeatable for every car, involving gentle acceleration (following any shift suggestions) and then constant driving at 60mph. On paper this looks odd as you might assume the figure would be taken at 70mph, but in truth this gives a more relevant real-world result taking into account the variation in speed that you get on a public road, and also the effect that the curved corners of Mira's speed bowl (which isn't a bowl so much as a triangle) have on economy. Over this test the 500 returned just 42.1mpg, even though it was done in Eco mode. It's not unusual for our test figures to fall short of the EU claimed economy, and there's certainly a pattern that the bigger diesel motors tend to get much closer to the stated figures than the small petrol engines. But this was an unusually large discrepancy so to be sure that we weren't doing the 500 a disservice, we gave the car to Production Ed, Tim Dickson, to do the 120-mile round commute that he normally does in a 2.0 TDI 143 Seat Exeo. He managed just 35.3mpg in the 500. The same test in the Exeo revealed that it was doing 56mpg. Now, clearly a two-cylinder 500 is a car that will spend most of its time around town. We don't do an urban-only economy test, but I spent a few hours in town in the 500 and in a completely unscientific test where I simply followed the shift lights and used minimum possible acceleration I got an indicated 50.1mpg, which I only managed to maintain for a few minutes before it dropped back into the mid-to-high 40s. There are a few points to make here. Firstly, are small capacity petrol motors really the answer that many manufacturers think they are? From my experience of various small petrol motors, I think that opting for a slightly larger, more powerful motor can actually be the more economical choice. The 120bhp 1.4 TSi VW Group motor seems to be the ideal balance between capacity and power, so that even under hard driving in the Golf, I was returning low 40s with ease. I’d recommend it to anyone, even if they did do lots of motorway miles. But the smaller, lower powered engines (diesel or petrol) need working harder to achieve the same performance, so economy suffers too much unless you're exclusively driving in town. I'd be delighted if Fiat can prove me wrong as its two-pot engine goes through further development, because it's a complete joy to use and if you don't mind the economy doubts then you should buy it anyway just because it's thoroughly entertaining. Interestingly, a knowledgeable bloke called Wolfgang Kuttler, who heads up Mini's diesel engine development, also told me that he believed there was a line where smaller capacity and lower power is less economical. He also reckoned that we'd only rinsed 50 per cent of the potential out of diesel motors, so watch this space for big improvements there. The real point to be made here is that in 99 per cent of cases the EU figures are simply not realistic. I'm all for makers stating a 'best' economy figure as an indication of what you can get out of a car if you drive behind a lorry on a flat road all day long. But it is time that the average figure was also stated that is genuinely achievable every day in normal driving. And regardless of how the figures are achieved, the testing methods should be made completely transparent to the public. Economy will always vary depending on a driving style and environment, but regardless of that in today's showrooms it's simply too easy to be misled by on-paper figures that are seemingly there to help salesmen rather than accurately inform buyers.