No, you don't get good mpg. The only advantage is favourable tax treatment so you get more miles per £, or € in your case. With some cars you can get a tank that replaces the spare wheel. Fitting a small fuel tank + LPG tank in the fuel tank space is very complex. Most of the time you still have to put up with a big drum tank. In the Duster you may be lucky because there is a huge space under the rear taken up unnecessarily by the exhaust silencer.