auto transmission gear ratio
Gm Launches More Fuel-saving Six-speed Automatic Transmissions
GM’s plan to offer one million vehicles annually with six-speed automatics by 2008 accelerates in 2006 and 2007 with nearly 40 global vehicle applications of the fuel-saving transmissions. Models of the 2007 Cadillac STS and SRX, as well as the Saturn Aura, Outlook and Pontiac G6, highlight the vehicles offered with new six-speed automatics.
Other models already on sale include the GMC Yukon Denali family, the Cadillac XLR-V, STS-V and Escalade lineup, and the Chevrolet Corvette. GM’s full-size heavy duty pickups are also equipped with an Allison 1000 six-speed automatic when equipped with the Duramax 6.6L turbo-diesel engine.
New 2007 model year six-speed automatic transmission variants include the Hydra-Matic 6T70 and 6T75 for front and all-wheel drive vehicles, as well as the Hydra-Matic 6L50 for rear and all-wheel drive vehicles. The 6L50 benefits from the expertise developed with the larger 6L80 rear-wheel drive six-speed transmission (initially in the 2006 model year), and is the second variant in the Hydra-Matic rear-wheel drive six-speed transmission family.
Advantages of a six-speed automatic include improved fuel economy, performance and a shift feel that is seamless to the customer. The six-speed automatic enables a reduced engine rpm at highway cruising speeds. A lower engine speed reduces engine wear and noise, and improves fuel economy.
“It’s a best-of-both-worlds scenario with the six-speed automatic, delivering great performance and enabling improved fuel economy,†said Jim Lanzon, executive director of transmission engineering for GM Powertrain. “It is almost like having two transmissions in one – the high numerical first gear provides tremendous off-the-line acceleration, but the transmission is able to use the six gears to evenly distribute the torque and settle at an overdrive gear that helps deliver great fuel economy.â€
Vehicle performance is enhanced with a six-speed, because smaller “steps†are used between gears, compared to a conventional four-speed automatic. This allows the transmission to quickly find the best gear for the vehicle speed and road conditions – there’s less back-and-forth hunting on grades, for example. GM’s new Hydra-Matic six-speeds also use technically advanced clutch-to-clutch operation, which reduces mass, complexity and enables compact packaging. A wide, 6.04:1 overall ratio on most of GM’s six-speeds, compared to 4.0 of typical four-speed automatics, delivers both great performance and improved fuel economy.
“The mechanical simplicity of the clutch-to-clutch shift operation in our new six-speed automatic transmission families, along with a conscious effort to reduce part complexity, is providing us greater flexibility and the ability to launch with a broad application across the GM portfolio,†said Lanzon. “By 2010, GM will introduce 10 new variants of six-speed automatic transmissions, producing three million six-speeds annually.â€
GM’s six-speed transmissions also provide tremendous flexibility to customize features for each vehicle brand, such as tapshift range selection, auto grade braking and a tow/haul mode.
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