Passing Fear: Do Fuel Economy Gains Compromise Quick Acceleration?: Scientific American
"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 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."