Electric motors produce maximum torque at zero RPMs, which means they snap to life at the touch of the accelerator.

FORBES: Electric Cars Will Change the World

The metric of RPMs takes into account both the number of passengers carried and the distances an airline flew during a given year.

BBC: Why is Atlanta the world��s busiest airport?

System-wide traffic at American Airlines increased 1.3% in June over the same period a year ago, hitting 11.4 million revenue passenger miles (RPMs).

FORBES: Split The Booty: American Airlines Takes International, US Airways Takes Domestic

Replacing RPMs on the left is a meter that lets you know how much power the car is pumping out, relating to how far you press the accelerator.

ENGADGET: Audi A3 e-tron hands-on (video) Hands-on

Mash the pedal in Auto EV mode and, after the electric motor pulls you off the line, there's a brief delay before the gasoline engine comes to life and the CVT raises the RPMs to maximize efficiency.

ENGADGET: Ford Fusion Energi plug-in hybrid test drive (video)