I dont own one but a friend of mine has a Frontera 2.5 petrol, gets 17mpg on petrol and abou tthe same on gas. Since gas is about 1/2 the price of petrol, he gets the equivalent in value as if he was getting 34 mpg (hope that makes sense). It needs to start on petrol but it can switch over at any time and you really cant tell the difference (Not sure if they ALL have to start on petrol or if theres a problem with this one).
I have heard of gassed diesels but never seen one - aparantly it still runs on diesel but once stared runs on a mixture.