The most efficient SUV is the Mercury Seattle at 56 MPG :)
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Volkswagen makes an L3 Lupo that advertises 78 MPG.
Hydrogen-powered cars such as the Honda FCX Clarity and the H2 Mazda Rx8 dont even use petrol.
http://www.fueleconomy.gov/feg/best/bestworstnf.shtml is the government website for fuel rankings and this is the direct link to the list of the most fuel efficient cars!
There are several places you can look online to find a list of the most fuel efficient cars. Check FuelEconomy.gov, they have a pretty extensive list. CNN recently did a story on the eight most fuel efficient cars. Check out their report.
The most fuel efficient cars can be compared using the motoring magazines but for comparisons tailored to specific vehicle usage, you cannot beat the comparison websites.
Any car that is labeled as a "hybrid" is going to be more fuel efficient. Some of the most fuel-efficient cars of 2011 are the Nissan Leaf, the Telsa Roadster, the Chevrolet Volt, the Smart car, the Toyota Prius, and the Honda Civic Hybrid.
The most fuel efficient cars on the road are smaller cars with either hybrid (gas/electric) or diesel engines. Examples of such cars are the popular Toyota Prius(44/mpg) or the Volkswagen Jetta(34/mpg) diesel.
The most fuel efficient vehicles on the market today are usually hybrids. However, many small and economy sized vehicles on the market are not hybrids but are very fuel efficient as well.
most deisle cars and smart cars. *please add to this answer I know there's more
Some of the most fuel efficient cars are the smart cars like 2013 Smart ForTwo, Honda Fit EV, and the Scion iQ. These cars are electric so the fuel they use is used to create the electricity they run on.
Energy efficient cars are cars that gives you the most amount of travel for each unit of fuel that they use. The difference from conventional cars is that they give you more travel per unit of fuel than conventional cars do.
The most energy efficient car available is the Toyota Prius. Please see the following, for additional information: www.thesupercars.org/top-cars/most-fuel-efficient-cars
There hasn't really been a brand of semi trucks recognized as the most fuel efficient. Since they run on diesel a more efficient fuel, semi trucks already are more fuel efficient than cars in terms of size and power.
There are many fuel efficient cars on the market. In 2012 the most popular cars included the Ford Focus Electric, Scoin IQ EV, Toyota Prius, Honda Fit EV and Ford Fusion.