GM Adding 8-Speed Gearbox to ’15 Large Trucks

General Motors confirms today plans to add later this year an all-new 8-speed transmission to a number of its large trucks, including the ’15 Chevrolet Silverado and GMC Sierra pickups and the GMC Yukon Denali and Yukon XL Denali SUVs, as U.S. fuel economy and carbon-dioxide emissions rules grow more strict in the coming years.

The gearbox, which many industry watchers expected last year with the launch of the redesigned large trucks, initially will mate only to models with the top-of-the-range 6.2L EcoTec3 V-8 engine.

GM also offers in its large trucks a 5.3L EcoTec3 V-8 engine and 4.3L EcoTec3 V-6 engine, each part of lineup of fifth-generation small-block engines that also bowed last year.

Sources tell WardsAuto the transmission likely will migrate to those other applications as production ramps up at the automaker’s Toledo, OH, powertrain facility. GM previously announced the transmission also would be outfitted for the ’15 Chevy Corvette and Corvette Z06.

GM boasts the in-house Hydra-Matic 8L90 transmission comes in smaller and lighter package than the Hydra-Matic 6L80 gearbox it replaces. One key element contributing to the tighter packaging, the automaker says, was moving the hydraulic oil pump from the center of the unit to underneath.

The 8-speed also features a 7.0 overall gear ratio, which is a wider spread than the outgoing 6-speed. GM says it provides a numerically higher first-gear ratio to improve off-the-line performance while pulling a heavy load or trailering. Its numerically lower rear axle ratios reduce engine rpm on the highway, the automaker says, which improves fuel economy.

GM does not release how much of an efficiency gain the 8-speed provides, saying additional details will come closer to the start of production in the fourth quarter. A 2-wheel-drive GMC Sierra with the current 6-speed mated to the 6.2L V-8 acheives 15/21 mpg city/hwy (15.7/11.2 L/100 km). Rated at 420 hp and 460 lb.-ft. (623 Nm) of torque, the combination can trailer up to 12,000 lbs. (5,443 kg).

Chrysler currently offers an 8-speed automatic transmission in its Ram pickups. Ford will add a 9-speed automatic transmission to its SUVs in ’15 and a 10-speed automatic to its F-150 in ’16, according to a WardsAuto forecast.

Automakers are using additional gears on transmission as one play to meet increasingly stringent fuel economy and emissions rules. In 2017 an automaker's fleet must achieve 35.5 mpg (6.6 L/100 km) and by the 2025 timeframe upwards of 54.5 mpg (4.3 L/100 km).

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