The Sonata Hybrid has a 166-horsepower, 2.4-liter four-cylinder engine and a 30-kilowatt electric motor that produce a combined 206 hp. This is a common setup for a hybrid, though the Sonata Hybrid's gas engine is a bit more powerful than many others.

Two aspects make Hyundai's system unique.

For one, it uses a lithium-polymer battery instead of the nickel-metal-hydride one found in other hybrids. Lithium-ion batteries are more efficient and take up less space; they're the type of batteries that power your cell phone and laptop computer.

Hyundai, though, uses an even more unusual lithium battery, called a lithium polymer. Basically, this type allows companies to tailor the size of their battery packs. The individual cells that make up the larger battery are license-plate shaped, but a little smaller and with a slight bulge in the middle. They're incredibly light, and while the Sonata Hybrid's battery pack did seem small, cargo space was drastically reduced versus a regular Sonata. The Sonata Hybrid has 10.7 cubic feet of trunk space, while the base four-cylinder model has 16.4 cubic feet. The standard Fusion has a similarly sized 16.5-cubic-foot trunk, but its nickel-metal-hydride battery reduces the space to only 11.8 cubic feet in the hybrid.

The Sonata Hybrid's six-speed automatic transmission is also a hybrid first. Hybrids typically use continuously variable automatic transmissions to maximize fuel efficiency. Hyundai preferred the six-speed, hoping it would provide a better driving experience, one that non-hybrid shoppers would be accustomed to.

This is probably why I enjoyed driving the Sonata Hybrid a bit more than either the Camry or Fusion hybrids, at least when it wasn't in its eco-friendly Blue mode.

When the Sonata Hybrid isn't in that fuel-sipping mode, shifts are crisp and acceleration is strong. It seemed to hustle faster than the base Sonata four-cylinder, though it was a bit sluggish compared with the turbo Sonata, which makes 76 more hp than the base.

My test drive took me through Southern California mountains, and the hybrid drivetrain performed well. The hybrid's handling and ride quality are both on the soft side, like the base Sonata.

Once you engage Blue mode, though, everything changes. The transmission didn't seem to know what to do on steep inclines, and the engine whined unsettlingly on hilly roads. Then again, hilly terrain isn't where you'd engage this mode; it would be of most use in city traffic and in either congested or wide-open highway stretches. The company says it increases efficiency 5 percent.

I was supposed to take a shorter route on my test drive, to make sure I caught my plane home, but the guidebook only had the longer route Ч a fact my co-driver and I discovered at the halfway point, when we swapped seats.

As he took the wheel to pilot us the rest of the way, I was worried we'd be late. Luckily, he was a heavy-footed chauffeur and didn't bother with the Blue mode. We hustled through the next 40 miles at a decent clip Ч up and down mountains, stuck in city traffic and on highways at higher speeds. That type of driving didn't throw our mileage numbers off as much as you would think. During my more moderate Ч but mostly uphill Ч 30 miles, we averaged 27.5 mpg. After his enthusiastic driving, the computer said our average was 31.8 mpg.

Before hybrid buyers scoff at these seemingly low figures, I'll say that these numbers should be considered the bottom of the Sonata Hybrid's mileage spectrum. Hyundai challenged journalists to eke out the best mileage they could on the route, and that meant some people rolled up the windows, turned off the air conditioning and drove as conservatively as possible. The winner of the challenge hit 60 mpg at an average speed of just 17 mph. I would not have made my plane at that rate.

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