These so-called flex-fuel vehicles can burn either gasoline or ethanol or almost any mixture of the two.

NPR: 'Flex-Fuel' Concept Fails to Deliver on Potential

In Brazil, 90% of new cars sold are so-called flex-fuel vehicles that run on either gas or ethanol.

FORBES: Big Oil Bets On Ethanol Too

There are an estimated five million flex-fuel vehicles but only about 500 E85 fuel stations and most of those in the corn belt.

NPR: 'Flex-Fuel' Concept Fails to Deliver on Potential

The government rates the fuel economy of flex-fuel vehicles at about 165% the miles per gallon (mpg) they would get on straight petrol.

ECONOMIST: Behind Detroit's sudden embrace of ethanol

When life-cycle costs are compared, natural gas is currently a more economical decision than flex-fuel powered vehicles, although both are more expensive than gasoline at current figures (keep in mind that these dynamics could change due to oil shocks or increased demand for traditional fuels both real risks).

CENTERFORSECURITYPOLICY: Energy Independence Now

One frequent question deals with Compressed Natural Gas or Liquified Natural Gas vehicles (Natural Gas Vehicles or NGV) in comparison with tri-Flex Fuel vehicles ( Gasoline, Ethanol, Methanol) vehicles.

FORBES: Governors Press Detroit To Make Natural Gas Vehicles