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NEW YORK (CNN/Money) - In 1981, the last time gas prices breached $3, adjusted for inflation, the average car got 21 miles to the gallon.

Jump ahead 24 years, a period when there have been huge advances in automotive fuel-efficiency, and the average passenger vehicle on the road gets...21 miles to the gallon.

In fact, however, vehicles have gotten considerably more fuel-efficient, according to a recent EPA report. It's just that car buyers have wanted other things from that efficiency rather than simply to travel farther on a gallon of gas.

For the same 21 miles per gallon, America drivers today are buying bigger, faster vehicles with a lot more power. Light trucks, a category that includes SUVs and minivans, now account for 50 percent of passenger vehicles sold in the U.S. That's more than double their share in 1981.

Could $3 gas change all that? There already are several signs that it is.

• Large SUV sales are 11.2 percent lower than they were at this time last year, according to AutoData.
• In the "beige book" released Wednesday, the Federal Reserve reported a shift to vehicles that use less gas.
• General Motors has indicated that it is extending its "Employee Price" incentives to include 2006 model year large trucks and SUVs, a sign that those vehicles need some sales help.
• These days, more consumers are saying that they are looking at gas mileage in making vehicle choices. In a recent poll by Kelley Blue Book and Harris Interactive, 59 percent of current vehicle shoppers said gas prices have either changed their minds or strongly influenced their decision.
• Meanwhile, according to Kelley Blue Book, resale values of used large SUVs are dropping.

At the same time, there has been an upturn in vehicle fuel economy in just the past few years. The reasons for that upturn are not yet clear, but as the market shifts back toward cars instead of trucks, that could allow fuel-saving technologies that are already commonplace to work in favor of actually using less gas.

If today's vehicles were the same, in terms of average weight and speed, as those in 1981, but had today's more fuel-efficient engines and transmissions, they would, theoretically, see a 30.5 percent improvement in fuel economy, according the EPA.

In 1981, passenger vehicles on America's roads had an average "ton-mpg" of 33.1 (ton-mpg is a measure of a fuel use compared to vehicle weight). Today, America's passenger vehicles get a "ton-mpg" of 43.2.

That's a huge increase in efficiency, but the average vehicle today simply weighs a lot more than it did then. In 1981, the average passenger vehicle weighed about 3,200 pounds. Today, thanks mostly to more SUVs and vans, the average vehicle weighs almost 4,100 pounds.

Besides the shift to trucks, a significant portion of that weight increase has to do with increased safety equipment and things like additional sound-deadening material to meet consumer demands for a quieter ride.

Vehicles have also been getting much faster. In 1981, the average vehicle had an estimated zero-to-sixty time of 14.1 seconds. Today, the average vehicle can hit 60 in 9.9 seconds. That figure includes SUVs and minivans. Top speed has also increased from 112 miles per hour in 1981 to 136 mph today.

Looking at those numbers, it's impressive that overall fuel economy has actually stayed the same since 1981. That's been possible because of a variety of technological advances involving just about every part of the modern automobile. Engines produce more horsepower for every liter of displacement. (Displacement is the total volume of space inside the engine's cylinders.)

Transmissions, particularly automatic transmissions, have more gears, which allows more efficient use of power.

Most of the efficiency improvements of the past couple of decades have come about by combining small improvements that, individually, would have been almost meaningless but, combined, have had a big impact.

"On the Hemi, adding the second spark plug was a 1 to 2 percent improvement," said Bob Lee, the Chrysler Group's vice president for powertrain design.

Overall, the basic 5.7 liter Hemi V8 engine is about 10 percent more efficient than the engine it replaced. The addition of "multi-displacement" technology, which shuts off four cylinders when their power is not added, as in steady highway cruising, boosted efficiency by another 20 percent.

Other technologies, like computerized fuel injection controls, multiple valves per cylinder, and lighter-weight engine parts, have also had a big effect, Lee said.

How that increased efficiency is used is largely up to the consumer. If consumers buy more efficient light trucks rather than more efficient cars, the effect on mileage will be muted.

As car buyers' tastes shift, prodded by rising gas prices, simple market economics could deliver the efficiency increases we've been missing. But to see that effect we would need a huge shift in consumer car-buying behavior.

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