How do you use your car to save money and save the planet? LPG.

Most medium-sized petrol cars can be converted to run on Liquid Petroleum Gas (LPG, or 'Autogas'). The process involves putting in a second tank - you can still use petrol if you run out of LPG. The tank goes in your boot, or can replace your spare wheel.

LPG is 20% cleaner than petrol, and has no particulates like diesel. And it's nearly half the price of petrol too. That's because there's less tax on it - the government actually wants us to use it. And if your car is fairly economical anyway, converting to LPG may take it into a super-low tax bracket, and may qualify for zero congestion charge.

I've just had my car converted. It runs just as well as ever. I fill up from the Shell garage in Park Street. A tank costs about £20. It cost me £1250 to get it done. On my mileage, I'll get that back in two years, then I'm saving hundreds of pounds a year.

The conversion company picked up the car and brought it back all done. It takes a couple of days, and they can give you a temporary replacement if you need it.

It's a total no-brainer. The only disadvantage is the initial outlay. After that you are saving money and polluting less too.