1 . Weight

Low weight is the cornerstone of a fuel-efficient vehicle because the amount of energy required to accelerate a car is directly proportional to its mass. Plus, lightness has a cascading effect. A lightweight body doesn't require a stout chassis, heavy-duty suspension components or large brakes. And the tires can be smaller, reducing rolling resistance.

2. Aerodynamic drag

The power—the rate of energy consumption—required to overcome aero drag is a product of the speed and the car's aerodynamic resistance. Lowering the resistance requires a two-pronged strategy: reducing the frontal area (the product of the car's height and width minus about 15 percent) and the drag coefficient, which is a measure of how easily the shape glides through the air.

3. Mechanical Efficiency

How much of the car's onboard energy makes it to the road? Electric powertrains are more efficient in this regard but require large and heavy batteries, which hurt overall vehicle efficiency. Internal combustion engines and liquid fuel are lighter but convert only about 30 percent of chemical energy to motion. Hybrids attempt to bridge the gap by capturing normally wasted braking energy.

4. Rolling Resistance

Drivetrain and bearing friction play a role here, but so does the rolling resistance of the tires. The less rubber on the road, the better, but beware of handling instability. And weight is the enemy of low rolling resistance (see No. 1, above).

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