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