Those empty promises...

One of sadly many conundrums I have failed to unravel in all the years doing this job is the reluctance of those who make high-performance cars to fit said machines with fuel tanks commensurate to that performance. Fitting a 500bhp supercar with a 17-gallon tank is akin to training Usain Bolt to the peak of physical fitness and then making him run in wellingtons. Yet still they continue.

You can work out a car's potential range by taking the size of its tank and multiplying that by its claimed fuel consumption figure. What you'll discover is a figure that, if trusted, will leave you at the side of the road waiting to be patronised by one or other of the roadside recovery companies. If treated with the total contempt this figure deserves it is, of course, almost entirely useless.

Let us take as one of many obvious offenders the Aston Martin DB9. Its fuel tank will hold just 17.6 gallons which, given its official combined consumption of 17.2mpg, gives it a maximum potential range of 302 miles. Except you'll never manage it, and for two reasons: first, no sane person is going to expect a fuel station to appear just as the tank runs dry. So let's say you'll typically fill up when the range meter says you have 50 miles left which at 17.2mpg is 2.8 gallons of tank capacity you're never going to use. Except these figures bear no relation to reality. Hardly ever have I matched any manufacturer's claimed fuel consumption figure over a tank of varied driving and, contrary to what you might expect, that's not because I spend my life driving like my pants are on fire. In fact it's the way these figures are calculated that leads to the misleading data. And don't blame the manufacturers for this: they're just doing what's mandated by European legislation.

If when driving your DB9 you actually average around 16mpg, so then multiply that by the tank capacity less what is now 3.1 gallons to represent the reserve into which you'll never intentionally dip, you'll discover your practical actual range is around 230 miles.

So here we have a fabulous Grand Tourer in the finest traditions that will barely travel the length of the M6 without needing to stop. And the Aston is just one of many examples I could have chosen.

It's the most regular complaint I hear from people who own such cars: they're not bothered by fuel costs, but they hate knowing every journey will be interrupted at less than three-hourly intervals.

And who can blame them? Service stations are broadly hateful places and I've borrowed enough flash cars to know how they attract attention, often of the kind you'd happily live without.

But they also spoil good journeys. I love having a drive of some hundreds of miles ahead. I'll make sure I have the provisions I need, then settle down to several hours on the road. You get into a rhythm, the time flies by, the distance remaining crumbling before you. Then you have to slow, stop, stand, queue and be subjected to some bovine shop psychology before you can head for the hills.

Point is, you don't want a fuel stop breaking your stride in any car. And in a GT or supercar, you want it even less.

So why should a basic Ford Fiesta carry you so much further than a Ferrari 599GTB? Why should this basic design failure mean a gently driven diesel-powered Focus takes no more time to drive from Calais to Berlin than a Lamborghini Murcielago going as fast as the autobahn allows (I know, I've tried)? Manufacturers will say it's down to packaging limitations, but GT cars must be the most space-inefficient on the road and I don't believe space cannot be found amid their vast expanses for a few extra gallons of gas.

In short it's a nonsense that must be costing sales. How many company execs who choose diesel cars do so because they're worried about the marginal fuel/tax costs of the petrol alternative, and how many do so because they know diesel is not just cheaper but quicker point to point and spares them at least one in three fuel stops? Tempting though powerful petrol cars seem on paper, in reality their case is hobbled by one easily fixed oversight.