Sometime in the next 12 months, Chevy will introduce the Spark, a 12-foot-long car that, along with the Fiat 500, looks like a rolling suitcase compared with the SUVs that populate our highways. Logically, you'd think that the main reason to drive such a small car would be: (a) that it's cheap and (b) that it gets great fuel economy. You'd be right about the first part, but a little off the mark about the latter.

While the Spark's official EPA figures aren't out yet, GM insiders have said that the highway number will be around 38—slightly lower than that of other larger cars in the lineup. Go up one size class to the Chevy Sonic and you can get 40 highway mpg. Another step up yields 42 mpg in the Cruze Eco. This scenario is not unique to Chevy. That Fiat 500 gets 38 mpg on the highway, and the new Scion iQ nets 37. Again, both figures are lower than in significantly larger cars.

Many aspects affect fuel economy. The biggies are engine size and efficiency, vehicle weight, and aerodynamic drag. How large a role each plays depends on how you drive. On the interstate, aero drag is the dominant player, and it turns out that a vehicle's length has a huge effect on drag. And counter­intuitively, short cars have higher drag than longer ones.

"The Spark's short length impacts the drag in two main ways," explains GM's small-car development manager, Dan Molnar. "First, the grille and windshield are more upright to allow greater passenger space, and second, there's simply not enough length to smoothly guide air around the car."

The drag occurs at both ends: the air hitting the front of the car, and the vacuum created as the turbulent airflow spills off the car's rear. Longer cars may weigh more, but they can better approximate a teardrop shape, resulting in slightly less resistance as they travel, which helps highway mpg. Engineers attempt to reduce drag on shorter cars with various body features, such as smaller wheel openings and sometimes a rear lip spoiler, but there's only so much that can be done. The Spark's drag coefficient is a few percentage points higher than the Sonic's.

So why drive small? Tiny cars weigh less, so in stop-and- go city driving they require a lot less fuel to get around. Where many 40-highway-mpg cars have much lower city-mpg figures (Hyundai's Accent gets 30 mpg, and the Focus SFE's figure is 28), the puny brigade typically fares far better. The iQ gets 36 mpg in the city, and the Spark should be close. Every car design is a compromise. The trick for those looking to save gas is, first, to figure out where you do most of your driving, and then to choose the car that best matches­ your profile.

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