Which is most fuel efficient car?

The most fuel effincient car is the Honda Hybrid.

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Hydrogen-fueled cars such as the Honda Clarity FCX and new H2 Mazda RX-8 don't even use gasoline/diesel.

Volkswagen Lupo 3L uses 78 miles per US gallon or 94 miles per Imperial gallon.

Fuel efficiency is often confused with fuel economy. If you drive a Toyota Prius like an angry teenager, you probably get worse fuel economy than if you drove a BMW 325i like a 45 year old schoolteacher, even though the BMW has far worse "fuel efficiency".