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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.
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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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