Volvo V40 Bi-Fuel

Say bi-fuel to most drivers and they will look confused and tell you to go and buy your own fuel. But there's a smug group of motorists who have already latched on to its real meaning - LPG at 40p a litre, against 68p for unleaded and even more for diesel. And that's not all; gas-powered cars are cleaner, so you will save a few pennies on the new CO2 company car tax from April.

So what's the catch? Inevitably it's not all good news, but Volvo has done its best to minimise the downsides of dual-fuel motoring with its new Bi-Fuel range. Even on the plain-Jane V40, the concept looks tempting. All the gas-powered models are built at the factory and have full warranties, unlike aftermarket jobs. There is an extra gauge on the dashboard and 'proper' switches rather than tacky add-on bulbs and dials. Meanwhile, the gas filler is housed neatly inside the standard fuel flap. You do have to sacrifice some boot space for the extra tank, but at least it's hidden under the floor.

Transferring between gas and petrol power is as simple as pushing a button on the dashboard, and the small drop in performance is barely noticeable. The only problem we encountered was a reluctance to start from cold when running on LPG.

But what we find less easy to fathom are those supposed savings. Burning petrol, the V40 will return an average of 34.9mpg, but gas gets only 27.2mpg, wiping out some of that advantage at the pumps. After leaving our office calculator smoking and notepads covered in long divisions, we reckon that the average 12,000-mile-a-year motorist will save