Big and thirsty and V-8 are out. Small and sustainable are in. Mini and smart are hot. Sequoia and Titan and Hemi, not so much. Industry strained financiallyA year ago, Detroit unloaded billions of dollars in health care obligations to the relief of many. Now, they're taking on $25 billion in government loans to retool plants to build more thrifty cars. Credit lines are being exhausted. Dealers are going bankrupt. Transaction prices are falling. How much can Detroit mortgage on the future -- with zero room for error? Not since the last oil crisis more than 30 years ago have U.S. automakers moved so fast to downsize their lineups. The second coming of the K car is among us. GM underwent an historic shift from rear-wheel drive to front-wheel drive, and from bigger to smaller, in record time in the 1970s. Names like Fleetwood and Roadmaster vanished. Now, the Jeep Grand Cherokee and Ford Explorer -- two truck-based relics of the 1990s -- are poised to become smaller, more car-like. The latest surge in gas prices has prodded Americans to shift gears big time -- and some experts see the change as permanent -- in favor of smaller engines. Hard to believe it took this long, but for the first time in generations, sales of vehicles equipped with 4-cylinder engines have spiked to more than 51 percent of the overall market in some months this year. Not since the Ford Model T put millions of Americans behind the wheel has the 4-cylinder engine been as popular. Trend reaches luxury marketYou know the market and consumer sentiment have shifted when luxury brands see salvation in small. Cadillac is engineering a more diminutive sibling to the CTS, complete with 4-cylinder engine, and hoping people have forgotten the Cimarron. Even BMW is poised to get religious about sustainability and fuel economy with 4 cylinder engines again. The Bavarian automaker is developing a turbo-charged 4-cylinder engine for the U.S. market to augment its venerable inline-6 cylinder offerings. For years Americans chose to visit a showroom and vote with pure lust and their oversized credit lines. Those days have abruptly ended. Credit is out, cash is king. We must pay as we go, and live and drive within our means. David Phillips is a contributor to Autos Insider and can be reached at [email protected].