Check out www.fueleconomy.gov for yearly information on all US cars fuel ratings. Any hybrid car (Honda Insight, Toyota Prius) will have excellent fuel ratings. Also, turbo diesel engines by VW do very well. Both the VW New Bug and the VW Golf models with TDI engines get 42/49 for the last few years.
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Hybrids have been proven to get poorer fuel economy than diesels in "real world" operation, especially in the relatively flat interior of Australia. Petrol hybrids are "in their element" in the cut and thrust of city driving, not on the open road.
For Fuel Efficiency, a Prius.
Fuel injection gives the best air to fuel ratio for best performance and efficiency.
Websites like fueleconomy.gov compares car with the best and worst fuel economy to help consumers determine the best car for them. Some good fuel efficient cars are 2009 Toyota Prius and 2011 Honda Civic Hybrid.
A car that runs on fuel cells only has a similar overall efficiency to a car that runs on petrol because a car that runs on fuel cells is designed to not pollute the Earth but you need renewable energy resources to generate the electricity which is like the efficiency of the petrol pollution. That was what I put on my homework anywayz. :)
check here http://www.wonderhowto.com/how-to-calculate-cars-fuel-efficiency-mpg-084812/
The 2014 Chevrolet Cruze delivers strong performance and a high-quality, roomy interior for a compact car and it has the most fuel efficiency.
Fuel efficiency for SUVs is not one of their strongest suites. At best for an SUV, you're looking at getting around 18 miles per gallon of fuel you put into the gas tank.
"The best affordable small car is the Hyundia Elantra. It combines a base price of $14,830 with fuel economy ratings of 29/40 miles per gallon city/highway and stylish design."
A device to modify air flow to increase fuel efficiency and handling
If not done right you can lose performance, handling and fuel efficiency.
The best rated and most fuel efficient vehicle known to man is the Toyota Prius Hybrid. This car is known to get upwards of 55 miles per gallon. It is strongly recommended by consumers.