The lowdown on LPG

You can buy an LPG-powered Falcon from as little as $1400 more ($2400 for a Commodore) and they offer the average motorist significant fuel savings.

But are they worth it and what are they like to drive?

We hope to answer these and other questions in the coming weeks because, after months of badgering, Ford has finally given us the keys to an E-Gas Falcon to put though its paces in the name of economy.

As the cost of fuel continues to see-saw, we thought this car would be of particular interest to readers as a low cost alternative to standard petrol engined cars and because LPG or Autogas as it's known is relatively cheap to buy.

Ford's Falcon is available in E-Gas form with XT, G6 and G6E grades _ all at a premium of $1400 each.

Our test vehicle is the middle of the range G6 E-Gas 4.0-litre automatic priced from $41,390.

LPG or Autogas as it is known is a combination of butane and propane gases stored under pressure in liquid form.

The two gases are generated as part of the oil refining process and generate less carbon than petrol _ 19 per cent less CO2 per kWh, 30 per cent less than coal and more than 50 per cent less than coal-generated electricity.

A third smelly gas called ethanethiol is added to the mix so that leaks can be easily detected.

LPG-powered vehicles can be distinguished by a small red sticker on their number plate.

Most readers will be familiar with the technology from taking taxis, 95 per cent of which operate on LPG.

The reason is of course is that their cheaper to run, but the impression is not always a favourable one.

It should be remembered however that taxis lead a long and very hard life and that the “taxi pack” as it's known in the trade sees the standard upholstery swapped for hard wearing vinyl.

Although LPG is cheaper to buy, it is important to note that you burn more of the stuff than petrol over any given distance.

For example, the Falcon G6 petrol model is rated at 10.5 litres/100km while our test G6 E-Gas is rated at 14.9.

In percentage terms, it uses an average 42 per cent more for every 100km travelled.

Because it burns more fuel, the LPG model is fitted with a larger fuel tank (93 litres versus 68) _ to give it the same sort of range.

But some space must be allowed for expansion, so the tank can never be fully filled.

The trade off for the larger tank is found in the boot where there's a bloody great spare wheel taking up valuable luggage space.

Looking at average fuel consumption for the Falcon, the petrol model has a theoretical range of 648km and the E-Gas, 624km.

At the time of writing, the price of unleaded petrol in Sydney was $123.9 a litre while Autogas was 64.9 cents.

Doing the maths, it means the petrol model will consume $13.10 worth of fuel per 100km and the E-Gas, $9.67 _ making the LPG model 26 per cent cheaper to run.

We can see the LPG model emerges as the clear winner, but there are some other considerations.

The Falcon engine produces less power when converted to gas and that the E-Gas model makes do with an old tech four-speed auto, instead of the current five or six-speed unit.

It's irritating but of less consequence than the fact the E-Gas model misses out on electronic stability control, a technology that has been proven to prevent single vehicle accidents.

Ford is quick to acknowledge this and it is working on the problem, hoping to offer DSC as it calls the technology with E-Gas models in the first half of 2009.

The 4.0-litre straight six E-Gas engine itself is basically a carry over from the previous model with some minor changes designed to improve economy.

New inlet and exhaust systems have been fitted, along with a re-tuned throttle map and optimised engine calibration.

With dual independent variable camshaft timing (DIVCT), stronger conrods, and harder valves and seats (the latter required for use with LPG), the E-Gas engine produces 156kW of power at 4750rpm and 371Nm of torque at 2750rpm.

In comparison, a standard petrol engine produces 195kW of power and 391Nm of torque.

Although it doesn't have the punch of the petrol engine, the performance penalty is not sufficient to outweigh the cheaper running costs of LPG.

And, after driving the car for several hundred kilometres, it really doesn't matter anyway.