Cut petrol prices – fill up your car for 75p a litre

Drivers switching to LPG can reduce what they pay at the pumps by 40%, but converting a vehicle costs £1,400. Is it worth it?

If the big jump in petrol prices has hit your budget hard, why not simply convert your existing model to run on 75p/litre liquefied petroleum gas, and slash your fuel bills by 33%-40%?

With petrol prices already nudging £1.37/litre in many towns, garages offering to convert cars to run on LPG are reporting a big increase in inquiries.

LPG doesn't give quite the same miles per gallon as standard unleaded petrol – typically 15%-20% fewer. But it produces lower C0₂ emissions – 10-20% less, while particulate emissions are almost zero.

Around 90% of the petrol cars on UK roads can be converted. It costs £1,200 to £1,600 for an average four-cylinder-engine model, and, once adapted, it will run on either petrol or gas. The installer will have to fit a secondary fuel tank in which to store the gas – this typically replaces the spare tyre – and make several changes to the engine.

Finding an LPG station is also becoming easier, although don't convert until you've established if there's a good supplier locally. There are around 1,400 UK garages selling LPG, which sounds a lot, but it's not available in every town. The UKLPG's website has a garage finder based on Google maps, and, of course, there's an iPhone app.

Although you will need to do the sums on your own vehicle, those driving a 35mpg car 14,000 miles a year will save more than £700 a year by switching to LPG – recouping the installation costs in around 18 months to two years. Less economical cars will recoup the investment quicker.

Mike Chapman of the UKLPG Association says technological advances in the last five years have ironed out the problems that some early LPG adopters experienced.

"The latest systems work with the engine management system, and are extremely reliable. Almost all petrol cars can be converted. Some of the newer direct-injection engines are not suitable, but to all other drivers, a correctly installed and maintained system will offer no problems. We know of many cars that have covered 250,000 miles on gas," he says.

He says a UKLPG approved installer – there are 200 across the country – will be able to explain exactly what your car requires.

Engines, he says, that are prone to valve and seat wear will find that this is exacerbated by running on gas. To get round this, installers now fit a lubricating system that adds around £200 to the installation bill.

Chapman runs a high-performance 3.0 litre Audi Quattro and a motorhome – both on LPG. The Audi's fuel consumption falls from around 26mpg on petrol to 21mpg on gas – a 19% loss. The price difference means a big saving.

On his system, the car starts on petrol and then switches to gas automatically after a few miles. His gas tank is in place of the spare wheel, giving him a range of 250 miles. Instead of a spare, he carries a foam tyre repair kit, although he says he hasn't had to use it yet.

Inside the cabin there is a fuel gauge showing the gas tank's level. If you run out of gas en-route, the car seamlessly switches back to petrol.

Remember, you will not be able to drive as far on a tank of gas as you would on petrol – some gas users will get as few as 150-200 miles from a tank – so you'll be filling up more often.

If you can't find gas, you just run on petrol as normal.

You also need to be comfortable losing the space that the tank takes up. Some drivers are happy to forego a spare wheel and use a repair kit. But if you had a bad blow-out that shredded your tyre, you'd have to wait for a tow truck. Another option is to carry a space-saver spare wheel. The system should be inspected each year and will need some minor maintenance which will cost £25-£50.

Drivers who convert their car to run on LPG have to tell their insurer. If you have used an approved installer, you should not see the premium rise, as converted vehicles are not considered any more of a safety risk in the event of an accident. And, because gas burns more cleanly than petrol, engine wear is reduced, so, in theory, your engine lasts longer on gas. Bear in mind, however, that your manufacturer's warranty could be affected.

Once installed, you are entitled to a small discount (£10 or £15 depending on the CO₂ rating) on your car tax. You need to apply at your local DVLA office.

The advantage of an LPG conversion is simply that it lets you keep your existing car, but makes it cheaper to run. But converting will only make sense if you plan to keep it for several years. Convert a three-year-old car, and run it on gas for five years, and you'll easily save £4,000 in reduced fuel bills – assuming the price differential continues – although this is by no means guaranteed. Alternatively, buy a used car that has already been converted.

This week Autotrader was selling an LPG-converted 2002 Vauxhall Astra with 210,000 miles on the clock for £849, and a 118,000-mile model for £1,450. These show that cars can run to big mileages on gas. There are plenty for sale – everything from Protons to Range Rovers.

Prospective buyers need to check the quality of the system. The registration number of all cars that have been converted by an approved installer are held on a UKLPG database.