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Auto Transmissions: How Many Speeds Do You Need?

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 for 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 a 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 happens in 10, 15 years," Mercedes-Benz's Manger said.

In the past, according to 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.

CVTs to the Rescue

Even if more ratios boost economy and efficiency, packing too many gears into a transmission case becomes logistically difficult, if not impossible. One alternative is to go all the way, to a continuously variable transmission (CVT). Operating with a special belt and a pair of variable pulleys, but no actual gears, CVTs deliver a near-infinite number of ratios. It's an old idea. Dutch-built DAF cars, sold briefly in the U.S. in the early 1960s, used such a system.

Several automakers have introduced CVTs in recent years. Audi offered a CVT in its 2002 A6 and A4 series. So did the then-new Saturn VUE and the MINI Cooper, though Saturn has since reverted to conventional automatics. Nissan continues to install one in every Murano. Ford's Freestyle wagon comes only with a continuously variable transmission. CVTs also are used in cars that have hybrid (gasoline/electric) powertrains.

A CVT has "less moving parts, less to go wrong," said Lexus' Hubbard. On the down side, some belts need to be replaced as often as every 30,000 miles or so. For its new GS 450h hybrid-powertrain sedan, Lexus has developed a variable-ratio transmission that employs a Ravigneaux planetary gear set, which provides two ranges: Low and High. A shift takes place around 60 mph, accomplished when a clutch restrains one of the internal gears. A similar dual-range system is used in the LS 460's eight-speed automatic.

Five-speed automatics aren't likely to disappear anytime soon, and CVTs are easing slowly into the marketplace. For now, at least, there's no need to feel deprived if your automatic transmission has fewer than seven or eight speeds.