How Efficient Can Internal Combustion Get?

What a surprise! People are reacting to stratospheric gas prices by turning their backs on SUVs and slipping into something, well, a bit cozier.

One in five vehicles sold in April was a compact or subcompact, up sharply from one in eight 10 years ago (when the Dodge Neon salesman was the loneliest guy in town). Meanwhile, sales of the Dodge Ram truck were down more than 29 percent, and the Chevy Silverado more than 30. The big F-Series pickup was once a big moneymaker for Ford, but it too is down 27 percent.

The Ford Focus is suddenly hot: sales are up 32 percent.

I've long dreamed about this day; our romance with gas guzzlers was a long national nightmare. Once SUV dealerships were glittering trophies, but now they're waiting for customers. I ran into a Jeep franchisee at a party last week and asked him how the brand was doing. He made a face and turned his thumbs downward. "They're great cars, but they use too much gas," he said. "My wife loves her Jeep, but it costs $65 to fill up."

So what's selling? The Toyota Prius is up 53.8 percent, and the Ford Focus up 32 percent. The tiny Honda Fit, which won high praise in a recent Wall Street Journal piece, is moving out of dealerships in just 20 days, selling even more briskly than the Prius.

The Fit is an economy champ, but is its combined 30 miles per gallon as good as it gets? I asked Norman Mayersohn, deputy editor of the New York Times' "Automobiles" section, what the theoretical fuel-efficiency ceiling might be for a conventional car capable of passing crash tests.

"I'd say the brick wall is around 50 mpg, and that would of course be very good," he said. "But to get there we'd have to resign ourselves to losing a lot of performance. We'd be talking about a car with a less than one-liter engine, taking 20 seconds to get to 60 mph and topping out at 75 on the freeway."

The Hypercar would use lightweight composite materials, but are they affordable?

Suppose we listened to "hypercar" guru Amory Lovins and started making cars out of ultralight composite materials such as carbon fiber? "It's still so expensive to use composites," Mayersohn says. "It works for luxury cars, but it would make small cars too costly. Also, fabrication is expensive and recyclability is bad or nonexistent."

All this explains why hybrids, plug-ins and electric cars are the talk of the town today. There's only so far we can go with internal combustion.