As if more evidence were needed of the recent tectonic shifts in the automotive marketplace, American car buyers have begun opting for vehicles with smaller engines in hopes of increasing fuel economy. In May, four-cylinder engines became the powertrain of choice for almost half of new car buyers, marking a near doubling of their popularity over the last four years.

The hard numbers are impossible to ignore. According to J.D. Power and Associates’ Power Information Network, 45.6 percent of retail buyers in May 2008 chose four-cylinder engines for their new car or truck. That number was only 38 percent in February 2008, and was at a low of 28.2 percent in May 2004. Today, less than 20 percent of newly purchased autos run on eight-cylinder engines, with six-cylinder engines also declining—both were passed in popularity by fours last month. J.D. Power says that May’s numbers represent the highest percentage of four-cylinder engines sold since it began keeping track.

The change in engine choice mirrors a strong market shift to smaller, more fuel-efficient cars, many of which only come with four-cylinder engines. Subcompact and compact car sales as well as compact SUVs are the only bright spots in a market already battered by a weak economy and high gas prices. This shift comes as the quality and selection of smaller engines has been getting better. Automakers are introducing direction injection, turbocharging and variable value timing to simultaneously boost power and fuel economy.

Given the lifespan of vehicles and challenging economic conditions, smaller engines are likely to continue becoming a greater percentage of U.S. vehicles in use. The market for hybrid gas-electric vehicles is steadily climbing, although only by around one percentage point a year. Meanwhile, the move to four-cylinder vehicles is viewed as a mainstream low-cost high-mpg option—ultimately with a greater reduction on national oil use and environmental impact.