The Battery Is Included, but the Benefit Is Slim MOST people aren't aware of this, but since the late 1990s the Chevrolet Malibu's success has roughly tracked the career of Robert Downey Jr. The early years of the millennium were not draped in glory for either the actor or the midsize sedan, but in 2008 Mr. Downey Jr. starred in "Iron Man" and Chevrolet released a seventh-generation Malibu that was finally ready to challenge the Toyota Camry. Think of the 2013 Malibu as "Iron Man 3," a follow-up effort aimed at keeping the acclaim rolling. On paper, the overhauled Malibu lineup looks strong. A new direct-injected 2.5-liter base engine puts out 197 horsepower, and the old V-6 is replaced by a stout turbocharged 2-liter 4-cylinder. But I haven't yet driven either of those, because the first 2013 Malibu to reach dealerships was the Eco model. And I'm not sure the Malibu Eco, a mild hybrid, makes a very strong case for its particular approach to efficiency. But before I dive into the question of whether to Eco or not to Eco, let's talk about the new Malibu platform. In nautical terms, the 2013 Malibu is what you call "beamy." She's wide. Compared with the previous Malibu, the new car's wheelbase is 4.5 inches shorter while its track (the width between the wheels) expands by more than two inches. But the overall length is almost the same, which means the inches excised from the wheelbase must have gone into the overhangs. That's perplexing, given that designers typically try to maximize wheelbase and minimize overhangs in the name of both aesthetics and interior space. I have a hard time imagining a chief designer looking at a styling proposal and proclaiming, "I hate how the wheels are pushed to the corners. Give me longer overhangs! The dimensional alteration could be related to the fact that the latest Malibu platform is destined for sale in nearly 100 countries, so the slightly shorter, significantly wider body may be related to international-type globalization factors, like whether Bhutanese passengers prefer to ride with arms akimbo. In any case, you lose a little bit of legroom compared with the old car. The Malibu interior is well turned out, especially when trimmed in the two-tone leather of the car that I drove. The seven-inch dashboard touch screen performs a neat trick, pivoting out to reveal a hidden storage compartment. Petty thieves, forget I wrote that. Chevrolet says the Malibu Eco is the quietest car it has ever produced, and I can believe it. The serenity is partly a result of sound-deadening tricks and partly because of the hybrid system's ability to keep the 182-horsepower 4-cylinder spinning just faster than the second hand on your watch. At stops, the gas engine shuts down automatically. If the powertrain were any more mellow, it would be hanging out in Neil Young's teepee. Yes, he has one. With its 15-horsepower electric motor contributing thrust, the Eco's low-r.p.m. wellspring of acceleration is more ample than you'd expect. The gas engine loafs, but the car surges on the extra torque provided by the electric motor - up to 79 pound-feet of it. Unlike a full hybrid, the Malibu Eco is never propelled solely by its electric motor. Sometimes, as you effortlessly chug up a hill, the Eco feels almost like a diesel. But that's assuming there is power stored in the modest lithium battery, with a capacity of just 0.5 kilowatt-hour, which is not always the case. I found myself frequently consulting the dashboard energy display to ensure I was topping off the battery, which meant trying to employ the regenerative braking system whenever possible. With regenerative braking, you get the satisfaction of sequestering energy every time you roll to a stop or head down a hill. The trick for engineers is blending regenerative braking with the conventional pads-and-rotor kind, which is still required for your more emphatic decelerations. The Malibu's regenerative system is fine when you're braking lightly and equally acceptable when you're stopping with vigor. But under moderate braking, the car sometimes can't make up its electronic mind, resulting in brake response that becomes annoyingly nonlinear. The sensation is that of an invisible driver-ed instructor constantly jabbing his auxiliary brake pedal, but with no real decisiveness. At one point during a lurching stop, a passenger actually asked, "Are you doing that?" Brake response has to be pretty ragged to attract the notice of someone who isn't even driving. In terms of both price and efficiency, the Malibu Eco attempts to split the difference between a conventionally powered sedan and all-out efficiency champs like the Toyota Camry and Ford Fusion hybrids. It's a worthy intention - to accrue a share of the full-hybrid benefits without all the cost and complexity - but the challenge is that conventional drivetrains are already optimized to wring the last inch of range from every drop of fuel. There's just no longer much breathing space between the best conventional powertrains and mild hybrids like G.M's. eAssist system, which made its debut in the 2012 Buick LaCrosse and is now offered in the Buick Regal and Malibu Eco. Nor is there much wiggle room on price, with the Malibu Eco's $26,065 sticker placing it within $655 of a Camry Hybrid. That is also about $1,600 more than a comparable non-Eco Malibu. On the nonhybrid side, the poster car for conventional fuel-efficiency excellence is Chevrolet's own Cruze Eco, which returns a government-rated 28 m.p.g. in town and 42 on the highway without any electrical assistance. Instead, the Cruze Eco uses a small turbocharged engine, a manual transmission and old-fashioned tricks like lightweight forged wheels. The Malibu Eco is a larger, more powerful car, but its mileage numbers are a long way off the nonhybrid Cruze, at 25 m.p.g. city and 37 m.p.g. highway. The Cruze Eco even has more trunk space than the Malibu Eco because there's no battery pack beneath the carpeting. While the Environmental Protection Agency hasn't released final numbers for the new base Malibu, I can't imagine that the Eco will beat it by more than 2 m.p.g. For comparison, the old 4-cylinder Malibu netted a combined city-highway rating of 26 m.p.g., while the 2013 Eco returns 29 m.p.g. combined. The Camry Hybrid beats both Chevys with a combined rating of 41 m.p.g. Even the 2013 Nissan Altima, with a conventional nonhybrid powertrain, improves on the Malibu Eco by 2 m.p.g., with a combined city-highway estimate of 31. The moral seems to be that if you're going to build a hybrid, with all the packaging challenges, cost and complexity that entails, you may as well go all the way. And if you're trying to take a less expensive approach, then the simple Cruze Eco formula - lightweight materials, small turbo motor - is effective and a lot more fun to drive. I suspect that I'll enjoy the other Malibus, particularly the 259-horsepower turbo model. But in Downey Jr. terms, the Eco is less like "Iron Man" and more like "The Shaggy Dog." INSIDE TRACK: An appealing car wrapped around a Rube Goldberg powertrain.