"It's not a hybrid! It's an electric car with a range-extending, gas-powered generator onboard." That was the party line during most of the masterfully orchestrated press rollout of what we've been promised will be the most thoroughly new car since, what, the Chrysler Turbine? The Lunar Rover? Well, the cat is now out of the bag, and guess what? It is a hybrid, after all. Yes, Virginia, the Chevy Volt's gas engine does turn the wheels. Sometimes.

On paper, the Voltec drivetrain has more in common with a Prius (and other Toyota, Ford, or Nissan Altima hybrids) than anyone suspected. Each system employs a single planetary gear set, a gasoline-powered piston engine, and two electric motor/generators. But the way Chevy connects them is entirely different, and—if you ask me—superior.

A planetary (or epicyclic if you're British) gear set consists of a solid "sun" gear, with teeth on the outside, a hollow "ring" gear with teeth on the inside, and some number of little planet gears rotating around the sun and also enmeshed with the ring gear. The planets themselves are connected by a "planet carrier." When you turn one of these three elements and stop a second one from turning, the third element turns at a different speed. By varying which elements are turned or driven and which are held, you can get three ratios out of one planetary gear set. If instead of stopping a particular element, you turn it at a different speed, you can achieve continuously variable ratios, as the Prius does.

Here's the huge fundamental difference: Toyota connects each of its electric motors and its gas engine to a different planetary element with the wheels and the bigger electric motor both connected to the ring gear. This means that, at some point in the gearing equation, the engine must turn on to move its element (the planet carrier) or the smaller motor/generator will start spinning too fast.

Chevy's engine and motor/generator both remain decoupled from the whole works most of the time.

The Volt's 149-horse electric motor spins the sun gear. When starting off, the ring is locked to the case and power flows to the wheels through the planet carrier, providing more mechanical advantage than the Prius' 80-horse electric motor gets driving the wheels directly. At about 70 mph, the Chevy's motor is starting to spin too fast to be efficient, so the ring gear unlocks from the case and locks to the smaller motor/generator. Now both e-motors spin, propelling the Volt to 101 mph turning at reasonable rpm in electric mode. The Prius' gas engine must start turning when vehicle speed exceeds 62 mph.

Once the Volt's battery is depleted, the engine fires up and clutches to the generator to produce the power required to drive the car. Above 70 mph, when the generator couples to the ring gear, the engine gets a more efficient direct mechanical connection to the wheels. In defense of Chevy's earlier stance, the only way this gas engine (or the Prius') could ever drive the wheels without lots of help from the battery is if you somehow MacGyvered up a way to jam the sun gear to a stop.