HOW does 80p a litre or £3.65 instead of £6 a gallon for your fuel sound? A blast from the past, a banker’s treat perhaps?

But it is feasible with an LPG conversion on a petrol car, such as being offered in the form of a new Mazda6 targeting business users but also available to the private buyer.

It’s a very attractive option with fuel prices up 50 per cent in the past six months or so and rocketing on, seemingly with no ceiling in sight.

Here at last is a chance to smile.

Okay let’s get one thing straight, you don’t get something for nothing. The conversion adds £2,000 plus VAT to the £18,845 list price of the car, but a business user clocking up say 18,000 miles annually should recoup that in under 18 months and then be quids in.

On the leasing front the monthly costs are on line to be very competitive against plain petrol or diesel and then you’re instantly on a winner with fuel bills.

And the host car is good, winning growing orders - among its chief competitors, the fleet award-winning Mazda6 is more popular than the likes of the Toyota Avensis, Renault Laguna, Honda Accord and Peugeot 407 although behind the Vauxhall Insignia, Volkswagen Passat and Ford Mondeo.

But its bonus now is the option of the LPG addition to conventional petrol running, and that is as easy as pie to use and the gas conversion is not a one-way change of combustion but an addition to the standard petrol injection system, providing cheap fuel but also an extended range.

The car starts on the key as normal, using petrol, and then within about half a mile imperceptibly switches to gas. And if you run out of gas it just as seamlessly switches back to unleaded from the 64-litre main tank.

There is no power penalty in terms of throttle response, although economy is a little lower than petrol, averaging around 35mpg rather than 42mpg, but hey, you’re saving about £2.50 a gallon!

Outwardly the only clue is an extra filler beside the petrol flap. In the cab just a button on the centre console which you press once for automatic operation, and on that are four green LED lights as a fuel gauge.

Under the bonnet there are four extra fuel feeds to the inlet manifold plus a feed system for extra valve protection.

And the tank? That’s purpose made, crash-tested and bolted into the spare wheel well, holding 70 litres or good for 400 miles or so.

The tank location means that the vast boot space is uninterrupted at 510 litres that can be extended to 1,702 by folding down the rear seats as normal – ideal for the business user or family.

What about a spare? Well to be honest many manufacturers are doing away with them because the weight adds to CO2 figures and they’re rarely needed so a repair inflation kit is provided instead.

LPG conversions started gaining popularity about a decade ago with the Government Powershift programme giving substantial grants, but then the cash dried up. And then the manufacturing industry started looking at petrol/electric hybrids.

But more recently LPG conversions have found new favour among those with larger-engined petrol cars like V8s and big 4x4s, so that alarmingly high fuel bills can be brought down to family hatchback levels.

But now that even those hatchbacks can swallow £100-worth at a fill with a 30-mile each way daily commute costing £60 a week or £3,000 a year, even they are getting a closer look, such as is now being promoted by Mazda with the Mazda3 and Mazda6.

And we’re lucky that the specialists doing the conversion, Prins Autogas UK, and also offering it as a retro-fit, are based here in at Bitterne in Southampton and backed by local Mazda main d e a l e r M a g n a M a z d a whose group chief executive Tony Roberts has been really impressed by the move.

He trained as a mechanic and said that he was struck by the professionalism of the installation and the way it did not intrude on the cabin or boot, which is why he is putting on a demonstrator.

He also pointed out wryly that the conversion was the cost of an optional satellite navigation system.

As the fuel prices continue to rise, knowing that you can afford to go where you want might gain some importance over a computer screen telling you how to get to a location that you might have found on a map!