I did some quick checking on edmunds.com and compiled a short list of examples ranging in price from $21k - $38k. These are all minivans or SUV's in either base or mid-level trim (in order to get the 6-seat capacity) and all with automatic transmissions.
Mazda 5 $21,345 21/28MPG 4-cylinder engine
Mitsubishi Outlander $22,995 23/28MPG 4-cylinder engine
Kia Sedona $24,900 18/25MPG 6-cylinder engine
Honda Odyssey $28,075 18/27MPG 6-cylinder engine
Toyota Highlander Hybrid $38,140 28/28MPG 6-cylinder hybrid engine
The hybrid is a little pricey, but I threw it in there for MPG comparison. The Mazda 5 is the smallest vehicle in this group, but has an impressive amount of interior room.
The question concerns 6 passenger car. None of the ones listed above are cars. The most fuel efficient 6 passenger car is the Chevrolet Impala with 3.5 Liter V6 rated at 29mpg highway. It must be special ordered with front bench seat.
The 2014 Chevrolet Volt is the most fuel-efficient small and mid-size passenger car.
The most dependable fuel efficient car of 2014 is the Ford Focus.
Toyota Highlander Hybrid
The most fuel efficient used car is the 2004 to 2006 Toyota Scion. It gets 30 miles per gallon combined.
Hyundai
diesel will be powerful
Click the link.
A car has to be fuel efficient. It means it has to completely burn the fuel.
The beetle is a fuel efficient car. 25 in the city and 32 on the highway makes is fuel efficient.
The Most Efficient Car is The Electric Car sports Car that recently came out. The last answer was terrible so i changed it but this one is probably worse.
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 vehicle on the road as of the present is the Toyota Prius. National statistics state that consumers are extremely happy with this vehicle.