Let's say a driver approaches a red light at 80 kilometers per hour (50 miles per hour) and coasts to slow down, then the light turns green and he or she floors the accelerator. As the car slows down, the transmission automatically downshifts into lower gears, but a sudden command to increase speed reverses that process and the transmission has to find the proper gear for quick acceleration. With new technology introduced in the past couple of years to meet upcoming fuel economy standards, drivers of a small handful of the latest Ford, Chevy, GMC, Volkswagen, Mitsubishi models may feel engine hesitation when they goose the accelerator, which is a source of frustration, at minimum, to many drivers. But is it more than an annoyance? Is it a safety risk as well? Carmakers say no.

"It makes noises that are different and feel different, but it's performing and working exactly like it should," says Richard Truett, Ford's power-train communications manager.

Nevertheless, Jonathan Linkov, managing editor of autos at Consumer Reports, says it can be "scary" if a driver doesn't feel immediate acceleration as the car tries to zip in front of traffic. Linkov says Consumer Reports found such performance issues when testing the brands noted above. So, carmakers are working to respond to complaints from consumers about the performance changes without compromising fuel efficiency.

The efficiency tweaks are part of an effort by automakers to comply with federal corporate average fuel economy (CAFE) rules that now require that the U.S. car fleet meet fuel economy standards of 35 miles per gallon (15 kilometers per liter) by 2016. One way that automakers aim to improve fuel economy on internal combustion engines is by altering the transmission, which can boost performance by an estimated 5 to 10 percent. Such transmission tweaks include reducing the engine's time spent operating at a very high rotational speed to lower mechanical friction, or reducing time spent with an open throttle to lower air friction in the cars cylinders. Any friction can result in wasted energy.

But those tweaks can come with compromises: jerky motions and abrupt shifts in low-speed driving. Carmakers acknowledge the transmission changes are to blame, but maintain they pose no safety problem.

Ford, for one, unveiled its new dual-clutch automatic transmission in its 2011 Fiesta and 2012 Focus. In a typical automatic transmission, high-pressure fluid is pumped through the works to change the gears. Ford's new transmission shifts gears instead via computer-controlled electronic actuators, a technology born in Formula 1 racing cars, which is a more efficient way to get the engine's power directly to the wheels.

The gear box in these transmissions is also about nine kilograms lighter, and thereby more efficient, than a regular six-speed automatic transmission, because a torque converter is no longer required to move fluid around to spin its input shaft. Truett says this system has resulted in fuel economy gains of 8 to 10 percent, compared with a four-speed automatic transmission.

The main trade-off for Ford, and all automakers now aiming to crank up efficiency for that matter, comes with the calibration—that is, how long the transmission is programmed to stay in an optimal gear for fuel economy, which is not the same as its optimal gear for responsiveness and performance, says John DeCicco, a fellow with the Michigan Memorial Phoenix Energy Institute at the University of Michigan in Ann Arbor. Higher fuel economy gains—on the order of as much as 12 percent compared with older automatics—come from gear settings that are more optimized for efficiency, not performance.

How much response one gets depends on what gear the car is in. When the transmission is calibrated for higher fuel economy, there is not as much response when the gas pedal is pressed, because it's effectively in a higher gear. That means the engine is turning more slowly. The slower the engine turns, the better the fuel economy.

The problem with selecting a lower gear is that the engine is running closer to its maximum power at that rotational speed, says John German, program director for the International Council on Clean Transportation. So to accelerate, the transmission must downshift. "Drivers like a fast response to the throttle and do not want to wait for the transmission to downshift before acceleration starts," he says.

This approach to engine efficiency has resulted in consumer complaints about driving performance. "We've seen an uptick in problems of engine stumbling, hesitating and automatic transmission shifting roughly," says Raffi Festekjian, director of automotive research for J. D. Power and Associates, a consumer products and services rating agency. Whereas this was the 10th most reported problem by consumers in 2010, this year it is the fourth highest, behind excessive wind noise; hands-free communication that does not recognize commands; and difficulties in using and seeing heating, ventilation and air conditioning controls.

Linkov, who test drove the 2012 Focus, says the driving experience at low speeds resembles the lurching feeling experienced as a passenger with a driver learning to pilot a manual transmission. A similar type of technology, in the Smart Fortwo, resulted in the same problem, he says, adding, "It's not what you'd expect from new technology."

Similarly, several drivers have issued formal complaints to the National Highway Traffic Safety Administration's (NHTSA) consumer complaint Web site https://www.safercar.gov/ about the "rough shifts" on their 2010 Chevy Equinox, equipped with General Motors's new six-speed automatic transmission. GM took a different approach than Ford in improving the its transmission efficiency, relying on a larger number of gears in combination with improved shift control to enable a smaller engine to do the work of a larger one and thereby achieve fuel economy gains. This enabled GM to replace a six-cylinder engine with a lighter, more efficient four-cylinder engine. More gears allow for a wider gear ratio coverage (higher first- and second-gear ratios) and greater torque multiplication in lower gears. This, together with improved transmission shift control, allows a smaller engine to feel like a larger one while accelerating. The transmission-related improvements boosted fuel economy by as much as 11 percent.

GM is also considering Ford's technology, because it offers weight benefits in eliminating the torque converter and thus fuel economy improvements, but "we would ensure it had good drive quality before we would allow it to sell," says Roger Clark, senior manager of the GM Energy Center. He says GM will make sure there is better control of the clutches and actuators that shift the gears, so that the gears respond in a way that pleases the driver.

Truett thinks Fords' mistake was in failing to educate consumers about their new system's behavior, so as to better manage customer expectations. Even so, he says engineers and software experts are trying to improve their systems in response to customer input.

A trade-off between performance and fuel economy may be inevitable in the long run, and as manufacturers change the performance of their vehicles to increase their fuel efficiency, consumer complaints about performance may not go away. Complaints about drivability (such as slow acceleration and rough shifting), however, are usually not indicative of safety issues, says an NHTSA source who declined to be identified. Should that agency receive complaints of potentially unsafe conditions, such as inability to accelerate, as opposed to slower-than-expected acceleration, it may warrant a safety investigation. "We believe that there is no need to reduce safety in order to achieve greater fuel efficiency, and that manufacturers will find ways to address both needs," the source says.