The new V6 from Aussie automaker Holden (a GM subsidiary) can switch from petrol to LPG without any noticeable loss in power. LPG is, of course, liquefied petroleum gas, one of the hot new fuel alternatives on the market these days.
The dual-fuel engine can be found in the Omega, Berlina, and Commodore models of the Holden line. That combination, of a trunk-mounted LPG tank and a hood-mounted petrol tank, is said to offer a range of 683 miles.
Putting this kind of engine in your vehicle will set you back AUD$3,900, although the government will hand you a rebate of AUD$2,000, making it a bit more palatable.