LPG

In message , Tom Sawyer writes

You can fit a 60 litre tank underneath at the rear, but it does involve fitting a special exhaust to re-route it out the side. With this method you don't lose the spare wheel or load space, although it's a bit more expensive.

Is it worth it? Well that depends on the projected mileage during your future ownership of the vehicle, and the respective price of LPG and petrol in your area. As a rough guide, take what you would spend on petrol at its current price, multiply by 40% and compare to the cost of installation.