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Automatic Transmissions: How Many Speeds Do You Need? Notes From the Road

Automatic Transmissions: How Many Speeds Do You Need?
By James M. Flammang/autoMedia.com

More choices, performance, and economy...

Not so long ago, a four-speed automatic transmission was still the norm. Four-speeds began with GM's Hydra-Matic, which debuted for the 1940 Oldsmobile. As more manufacturers adopted automatics during the 1950s, many initially had only three speeds, or even two. Eventually, most automakers moved up to four.

Early in the 1990s, five-speed automatics came along, initially on top-end Mercedes-Benz and BMW models. Six-speeds arrived in 2002 for the BMW 7 Series sedan, and in the redesigned 2004 Audi A8. Mercedes-Benz skipped the six-speed step and shifted directly from five ratios into a seven-speed in 2004. Now, Lexus has upped the ante to eight in its 2007 LS 460 sedan. Can nine- and ten-speed transmissions be far behind?

More Gear, More Miles
Some observers suggest that "bragging rights" tops the list of reasons to issue transmissions with more ratios. Many believe it's largely a marketing measure, to draw attention to the company. Still, there are practical reasons for expanding the number of ratios, especially to improve fuel economy. Without question, today's automatic transmissions are more economical than their predecessors, achieving fuel-consumption figures that are a lot closer to what can be expected from a manual gearbox in the same vehicle.

With seven gears "you have much better variability" between low and high road speeds, said Dr. Stephan Manger, senior manager for overall vehicle development of the Mercedes-Benz M-, R- and GL-Class. "Intelligence in the system recognizes which is the correct gear." A five-speed transmission might have only one or two choices to downshift into when needed at a given road speed. Additional gear ratios present additional choices "to get to the right point."

Being able to say you're the "world's first" with an eight-speed automatic offers some promotional advantages, said Lexus product education administrator Charles Hubbard. Speaking tangibly, though, "the closer I get the ratios, the more I'm saving on fuel." Could engineers stuff nine or ten gear ratios into an automatic? "They probably could," Hubbard acknowledged. Is there a practical limit? "Nobody knows what can happen in 10 or 15 years," Mercedes-Benz's Manger said.

In the past, said Jeff Baran, chief engineer for rear-drive six-speed transmissions at General Motors, developers had to "trade off" between responsiveness and gas mileage. "Wider overall ratio transmissions," such as GM's new six-speed unit, can "enable performance and fuel economy in the same package." Depending on the vehicle model, GM claims "up to six percent improvement in some performance numbers, three percent in fuel miles per gallon." With two overdrive gears and a wide (6.04:1) spread of ratios, GM's transmission is said to approach the functionality of a seven-speed. Some five-speeds "didn't have a large difference in the overall ratio coverage," Baran said. Moving to the six-speed has also "reduced the mechanical complexity" inside the transmission, using fewer clutches and freewheelers.

Excessive gear changing is a potential drawback of an automatic with too many ratios. If the unit shifts up and down too often, drivers and passengers can easily get annoyed. Mercedes-Benz's Manger admits that with "too many gears, the shifting numbers are too high. You come to the point where the car shifts too often." Automakers rely on the electronic control system to keep the number of gear changes at a reasonable level.

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