I read recently about a fantastic Aussie invention by Perth-based company, Eden Energy that cuts the cost of running a diesel by using LPG. It sounds like a winner, both for the environment and economy, as I have a 15-month-old Hyundai Santa Fe and while the mileage is pretty good to have a bolt-on dual-fuel kit that cuts consumption of diesel by 70 per cent is amazing, especially as it's an Aussie invention.
There are many dual-fuel systems available for diesels and they all work by substituting LPG for diesel, and so cutting the cost of running your vehicle. While they generally work well, they must be installed by a reputable company with lots of experience in running a diesel engine on LPG. Many people find they don't get the savings they think they will, and some have damaged their engines, because they can't resist using the extra power the engines develop on LPG. Instead of backing off and using less fuel they keep their foot in it and don't get the savings, and work their engines harder at the same time. If you are considering such a system find someone who has done the conversion and get their experience first hand. You should also be aware that your warranty would be affected should you fit the system.