This is not the story we intended you to read. It started life as an attempt to prove Citroën’s contention that, if driven gently in an out-of-town environment, its new diesel-powered C1 supermini would return 100mpg. That would make the C1 diesel not only the most economical car on sale but also the first to stretch a gallon of any fuel over a three-figure mileage.

Indeed, we brought a petrol-powered C1 along only as a way of putting the diesel’s achievement into perspective and highlighting the yawning gap that, if you believe official figures, exists between the consumption of petrol and diesel engines.

In the end, however, instead of playing a supporting role it was the petrol C1 that stole the show. It achieved 85.5mpg, putting it well ahead of rivals and, in the process, casting doubt on the wisdom of buying a diesel car at all, particularly at the budget end of the market.

The official figures provided by Citroën state that the C1 1.4HDi will return 83.1mpg on the extra-urban cycle, making it by this measure the most economical car on sale. In fact it is the only car in Britain which, according to figures endorsed by the Department for Transport, has an (extra urban) mpg figure of over 80. Citroën reckons even that figure is conservative and is touting 100mpg as more realistic.

With petrol power, Citroen’s statistics say that the C1 1.0i can stretch a gallon of unleaded to 68.9 miles, which to most would prove the diesel is a wiser buy, especially at current fuel prices. In fact the only thing it proves is that you can prove anything with statistics, particularly if they’re not generated in real world conditions.

By contrast our test could hardly have been more real world. We filled the cars until they would take not one more drop, did a lap of the M25 in convoy, and filled them again.

Why the M25? Because it is the most used road in the UK and, some claim, Europe. Better still, because it is circular you can start and finish at the same point, meaning the results are not skewed by wind or gradient.

Although we tried to keep the cars to the 50mph Citroën recommends for maximum economy, this was a heavily trafficked public road and not a computer simulation or a rolling-road test bed. Nevertheless, we were keen to allow the C1 to give its best shot, so we kept off the brakes as much as prudently possible, accelerated gently, never let the engines labour and sweltered with the windows up and the air-conditioning off (air-conditioning can increase fuel consumption by 10%). We could have folded in the wing mirrors and coasted downhill but we thought that was taking things a bit too far.

We returned to Clacket Lane services, 2hr 23min after we had left, having averaged 49mph for the 116 miles we travelled round the M25. And we were greeted by two very distinct surprises.

First was that the diesel C1 got nowhere near 100mpg. In fact it swallowed 6.38 litres of diesel, giving it an overall consumption figure of 82.7mpg, astonishingly close to the 83.1mpg claimed by Citroën on the extra-urban cycle. But our jaws didn’t really drop until we refilled the petrol-powered C1.

Despite having a more powerful engine (68bhp v 55bhp) than the diesel, we could not get more than 6.17 litres into its tank, despite spending an age adding fuel almost by the drop to make sure it really was truly full. This meant the petrol C1 had not only beaten its diesel sister, but smashed the official claim of 68.9mpg.

This was, of course, just one test, conducted entirely on a motorway with the specific aim of extracting maximum economy from both cars. Even so, the test does call into question the wisdom of buying a small diesel car particularly when, in the Citroën’s case, the £8,445 C1 diesel is £1,100 more expensive than the identically specified petrol C1.

You’ll pay more to insure the diesel and, typically, more to put fuel in, too. Yes, it is likely that the C1 diesel will depreciate more slowly than the petrol version but it nevertheless seems hard to construct an argument in favour of the diesel — it doesn’t even emit less CO2 — particularly when you factor in how much more enjoyable (and quicker) the petrol C1 is to drive, with its fizzy 1 litre three-cylinder engine relative to the comparatively rattly 1.4 litre four-cylinder diesel.

That said, we still reckon this was an amazing achievement for both of the little Citroëns. Now that the highly specialised Honda Insight hybrid is no longer officially on sale in the UK, we know of no other car likely to eclipse the petrol C1’s economy in a typical driving environment. For sure you could expect the C1’s clones, the Toyota Aygo and Peugeot 107, to be identically economical (though neither is yet available here with diesel power) but others, if official figures can still be believed, come nowhere near. Fiat’s diesel Panda, with 76.3mpg on the extra-urban cycle, and VW’s diesel Lupo with 78.5mph, run it closer than most. The much-vaunted Toyota Prius hybrid scores only 67.3mpg.

In the end it cost just £6.10 to lap the M25 in a petrol C1 and £6.37 in the diesel. Based on extra-urban consumption figures and fuel from the exorbitant Clacket Lane services where diesel or unleaded is nearly £1 a litre, a 1.6 litre petrol-powered Ford Focus would cost £10.24 to complete the same journey. To get around London in a Bentley Arnage would cost £27.57.

Whether you think the saving represented by the C1 is worth it is another matter. Travelling at the same speed as lorries, we lost count of the number trying to bully us out of their way. And when you drive something as little as a C1 with your entire rear window full of Dutch heavy goods vehicle, you suddenly understand the meaning of intimidation.

Still, until another manufacturer invites us to prove otherwise, we reckon Britain’s most economical car is neither diesel nor hybrid but a conventional petrol-powered hatch. Which was worth even the tedium of lapping the M25 to find out.