When the first fuel crisis hit America in the mid-'70s, customers immediately demanded more fuel-efficient vehicles. It didn't take long for the auto industry to respond. According to a study of automotive fuel economy and technology recently released by the EPA, the fuel economy of cars and trucks jumped from 13.1 mpg in 1975 to 21.1 mpg by 1982.

That improvement was driven by a substantial downsizing of both cars and trucks, a shift from rear-drive to front-drive cars, more efficient transmissions with lockup torque converters and more gears, and the incorporation of electronic engine controls. A 61-percent mpg improvement over seven years is impressive by any standard.

Then it stopped. After achieving 21.1 mpg in '82, the fuel economy of cars and trucks sold in subsequent years fluctuated between 20.5 and 22.1 mpg. For '06, the EPA expects an average of 21.0 — right where we were 24 years ago.

How can this be when the technological upgrades have continued unabated in the intervening quarter-century? Most engines have gone from two valves and pushrods to double overhead cams with four valves. Transmissions have continued their ratio proliferation, with six-speeds commonplace and seven- and even eight-speeds appearing. Computer-controlled fuel injection has become the standard on everything. Tires roll with less resistance. Aerodynamic drag coefficients are vastly lower.

In the face of these substantive, across-the-board improvements, why has fuel economy stalled for a quarter-century? The answer begins with the choices U.S. drivers have made.

Interestingly, the report documents that car mileage has actually improved since 1982, rising from a combined, adjusted EPA figure of 22.2 to 24.6 mpg estimated for 2006. The corresponding truck figures have also risen, although not as much, from 17.4 to 18.4 mpg. The problem is that we're buying a lot more trucks than we used to.

In 1982, cars accounted for 80 percent of the market, leaving only 20 percent to pickups, SUVs, and vans. By 2004, cars had slipped to 48 percent of the market and trucks had risen to 52 percent. Even this year, despite high gas prices, there has been only a modest shift back toward cars, which are barely 50 percent of this year's sales. Meanwhile, pickups comprise about 15 percent, vans 8 percent, and SUVs have 28 percent. This enormous shift toward larger and thirstier vehicles has kept fleet fuel economy down, even though the individual vehicles have improved.

In addition to buyers shifting to trucks, the vehicles have gotten heavier and larger. Back in '82 cars had downsized enormously, weighing 3054 pounds on average. That weight was down a full 1000 pounds from 1975, which is as far back as the EPA's data go. But by 2006, cars had packed 500 of those pounds back on again.

For trucks, the weight gain is even larger. Current trucks average 4712 pounds, up 900 since '82, and even 650 pounds higher than they were in 1975. Modern full-size pickups dwarf their predecessors of three decades ago. Moreover, these jumbo pickups comprise about 85 percent of today's pickup sales, up from about half of the segment in 1982. Those cute little Chevy LUV mini-trucks and their Toyota and Nissan equivalents are either gone from the market or they've swollen to the size of the full-size pickups of yesteryear.

SUV sales have exploded since 1982, and the segment has shifted toward larger models. Van sales have gone the other way, with minivans becoming dominant. Even so, today's long-wheelbase "minivans" dwarf a mid-'80s Dodge Caravan.

This weight increase hasn't all been caused by vehicles becoming larger. Cars, for example, have stayed fairly constant in size over the years, but they've grown denser. While EPA interior volume has grown from 106 to 112 cubic feet since 1982 — an increase of six percent — car weight has ballooned almost three times as much, or 17 percent. Blame stricter safety standards and customer requirements for stiffer, more-rattle-free vehicles and ever-increasing demand for creature comforts.

These larger and heavier vehicles need more power to haul themselves around, and that has happened, in spades. The year 1982 was the nadir of average horsepower for cars at 99, down from 136 in 1975, according to the EPA. In 2006, that average power has doubled from the 1982 figure to 198. The truck increase is much the same, going from 120 horsepower to 239.

The EPA has estimated 0-to-60 acceleration from these tables of annual average power and weight and come to the conclusion that every reader of this magazine already knows: Today's vehicles are the fastest in history. According to the EPA, the average car's 0-to-60 time has improved from 14.4 seconds in '82 (it was 14.2 seconds in '75) to 9.5 seconds today. The truck improvement is almost identical.

Our track tests suggest these numbers are on the slow side, but the magnitude of the improvement is about right. After all, when V-6-powered Honda Accords and Hyundai Sonatas can hit 60 mph in the mid-sixes, power is not in short supply.

With modern vehicles so much heavier and faster than they were in 1982, and with a steady shift toward the heaviest, least-efficient vehicles, it's no surprise that fleet fuel economy has stagnated. But what if cars and trucks hadn't gotten so much heavier and faster?

The EPA report suggests that fuel economy varies directly with vehicle weight. Therefore, if the average '06 car went on a diet, dropping from 3563 pounds to its '82 weight of 3054 pounds, its fuel economy would jump from 24.6 to 28.7 mpg. Losing no weight but dialing back performance from the '06 level to 1982 would achieve an identical fuel-economy improvement. Losing both the weight and the power would boost fuel economy to 33.5 mpg. With trucks, the results would be similar.

U.S. buyers will determine how much of this happens. Although the current high gas prices have motivated some shifts away from the largest trucks and SUVs, buyers have not abandoned them en masse for tiny econoboxes. Even hybrid sales are starting to slow, with reports that Toyota Priuses are being discounted. Once you've gotten used to plenty of power and room, it's hard to go back to cramped and slow.

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