DETROIT — Back when gas was cheap, Americans bought cars with V-8 engines like the Big Block, Cobra Jet, and Ramcharger. Acceleration was all that mattered, even in family cars that never made it to full throttle.

The 427-cubic-inch Chevrolet Tri-Power was the siren song of the gearhead, sending Corvettes roaring down the highway at up to 140 miles per hour.

But now, thanks to government regulation and gas-price gyrations, the motors that move the nation’s cars and trucks are shrinking.

Whether they drive hulking pickups or family sedans, Americans are increasingly choosing smaller engines that use less fuel, especially four-cylinder models that offer more horsepower than was possible just a few years ago.

More than half the new cars and trucks sold in the United States through May had four-cylinder motors. That’s up from 36 percent in 2007, and it’s the highest sales percentage since 1998, when the J.D. Power and Associates consulting firm started keeping track.

The smaller engines are helping to change America’s gas-guzzling ways. The government now requires automakers to build more fuel-efficient cars and trucks. Drivers are eager to save money on gas, which recently flirted with $4 a gallon and is still pricey at an average of $3.53. People have also embraced cars with downsized engines because new technology has made them just as fast as older cars with bigger motors.

In general, car shoppers can pick from three types of engines: four-, six-, and eight-cylinders. More cylinders usually produce more horsepower but also burn more fuel.

Decades ago, when gas was cheap, buyers usually went for bigger engines to get more power. Back then, noisy ‘‘fours’’ clattered down the highway inside compacts or wimpy midsize cars. Some drivers complained that four-cylinder cars didn’t have enough power to merge safely onto busy highways.

That began to change in the 1990s, when Honda and Toyota refined their fours, making them quieter and more powerful. In 2005, gas prices spiked after Hurricane Katrina knocked out refineries. The steeper prices made fuel-efficient cars more popular and forced Detroit’s truck-obsessed automakers to spend more money improving their smaller engines.

Small engines got another boost in 2007, when the government began raising gas mileage minimums, eventually requiring new cars and trucks sold in the United States to average 54.6 miles per gallon by 2025. The shift toward smaller engines gathered more momentum in 2008, when gas spiked again — above $4 a gallon.

Because of technology advances, many four-cylinder engines are more powerful than V-6s from only a few years ago. For example, today’s Hyundai Sonata midsize car has a 2.4-liter four with 198 horsepower, 45 more horses than the base V-6 in a 2006 Ford Taurus.

To boost the efficiency and power of small engines, companies have introduced all kinds of technology:

■  Direct fuel injection is more common. It mixes air and gas in the chamber that surrounds the piston, helping produce more power, more efficiently.

■ Many small engines now have turbochargers, which force high concentrations of air into the piston chamber, allowing more gas to be sent in and offering extra acceleration or hauling capacity when drivers step on the pedal.

■ Engineers have made cars more aerodynamic. Some vehicles shut off their engines automatically at stoplights. They can run pumps and other devices off the battery rather than a belt that sucks power from the engine.

Even as they become more powerful, smaller engines are helping lower gas consumption. So far this year, consumption is down 5 percent from the same period a year ago, according to government data.

The average new car goes about four miles farther on a gallon than in October 2007, said Michael Sivak, a research professor at the University of Michigan Transportation Research Institute. In March, the average mileage of new cars hit a record 24.1 miles per gallon, dropping slightly since then.

The improvements become more striking when drivers compare engines. Four-cylinder engines averaged 26.4 miles per gallon this model year, compared with an average of 16.1 miles per gallon for eight-cylinders, Sivak said. If gas were at $4, the average driver would save roughly $1,300 a year by switching to a car with the smaller engine.