We’re somewhere on the A343 in Hampshire. The road snakes quite steeply downhill and we’re building speed rapidly – 45mph, 50, 55, then 60mph.

Up ahead are BTCC aces Matt Neal and Gordon Shedden, driving a Honda Civic Tourer liveried up just like their race cars, and we’re slowly gaining on them. In neutral.

There’s a Volkswagen Passat between us and them and, for a moment, I consider overtaking it to conserve our precious momentum. Suddenly, the racers have to brake almost to a standstill for a van in front that has slowed to turn right. It’s infuriating.

We’re only a few hundred metres from our next destination and we could have got there without touching the accelerator pedal had no one got in the way.

This, if the pictures above haven’t already given it away, is the MPG Marathon. It isn’t a race – far from it – but the competition is white hot.

Despite this, my co-driver Kevin Booker and I reckon that we could be in with a chance of doing pretty well, given the outstanding real-world economy of the 1.6 i-DTEC Civic hatch we’re driving and what turns out to be a shared long-term preoccupation with particularly parsimonious driving.

The annual MPG Marathon has been running since 2000, when Ross Durkin of the CO2-savvy Fleet World magazine decided to resurrect the Mobil Economy Run, a fuel economy driving challenge held a few decades ago.

It was contested by eco-driving pioneer, one-time economy world record holder and former Autocar deputy editor Stuart Bladon, who, incidentally, is still at it, driving a Peugeot 308 in this year’s event with brother Hugh as co-driver.

Durkin’s timing for the inaugural MPG Marathon was fortuitously impeccable, coinciding as it did with the UK’s near-crippling fuel price protests, which made almost everyone in the country acutely aware of just how much fuel their car was using.

Since then, because of environmental concerns/wars in the Gulf/punitive road and company car tax rules/spiralling fuel duty (delete as appropriate), fuel economy has gained an ever-increasing importance.

These days, the MPG Marathon, which takes place on public roads, takes the form of a ‘navigational scatter’ event. In other words, competitors are given a list of destinations to which they can get by any route they so choose.

A few years ago the routes from point to point were fixed, but this caused all sorts of logistical and administrative headaches with the organisers because it meant that the event was classified as actual motorsport.

The ‘scatter’ approach is much simpler, plus it allows drivers to get creative with their navigation. To prevent cars becoming too much of a low-speed rolling road block along the way, there’s a maximum time allowed to get to each of the day’s checkpoints, with half a mile per gallon docked from your overall recorded economy figure for each minute you’re late.