Why Do Subcompacts and Compacts Get Such Similar Mileage?

No longer is 40 mpg only the province of the hybrid.

With four new members from the New York International Auto Show — the Honda Civic, Hyundai Accent, Kia Rio and Mazda3 — the crop of 40-plus mpg, non-hybrid gasoline cars available will creep into the double digits by the end of the year. You don’t necessarily have to downsize to get the best mileage: The 2012 Ford Focus SFE will be rated 28/40 mpg city/highway, while the pint-sized Ford Fiesta SFE gets 29/40 mpg. The entry-level Accent gets 30/40 mpg; the Elantra, its larger sibling, is rated 29/40 mpg. The stick-shift Chevy Cruze Eco gets 28/42 mpg, and GM expects the forthcoming Sonic to secure a 40 mpg highway rating.

In most cases, the compact cars are significantly larger, heavier and more powerful than their subcompact siblings. Why then is their gas mileage so similar? At the auto show, I posed the question to automakers and analysts. The simple answer: aerodynamics.

“Keep in mind that technology will help both [highway and city mileage], but not at the same rates,” said Mike O’Brien, Hyundai’s vice president of product planning. “The city number is going to be the bigger difference when you go a class up in car size.”

Conversely, the fight for better highway mileage goes against what O’Brien calls “road load,” a combination of the vehicle’s frontal size, tire friction and drag.

In some cases, it’s actually harder to eke out better highway mileage in a subcompact car, said IHS Global Insight analyst Aaron Bragman.

“When you have a B-segment [subcompact] car, it’s very hard to make it aerodynamic, given how short it is and how many of them end abruptly,” Bragman said.

That’s not to say carmakers aren’t trying. Cars like the Accent and Fiesta use technologies commonly reserved for luxury models — a direct-injection engine in the Accent’s case and a dual-clutch six-speed automatic in the Fiesta’s.

At Ford’s auto-show stand, marketing manager Robert Parker noted the incremental nature of this technology.

“Our engineers like to say fuel economy is about hundreds of little things,” Parker said. “All of those hundreds of little things add up to a tenth [of a mpg] here and a tenth there.”

Put another way, major increases in mileage — 30-mpg pickup trucks or 50-mpg commuter cars — will take more than slipstream aerodynamics or six-speed automatics.

“A lot of these countermeasures are low-hanging fruit,” Bragman said. “Long-term, you’re looking at a different powertrain, a different type of propulsion method, to get to the next level of fuel economy.”

The real question isn’t technology, Hyundai’s O’Brien said. “There’s always more technology,” he said. The real question is who will pay for it.

“You can have the cleanest car in the world, but unless someone buys it, you can’t be clean,” he said. “It’s really a tipping-point discussion.”

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