Navigant Research Blog

Automakers Add Gears for Better Fuel Efficiency

David Alexander — September 3, 2014

Automakers are pursuing many options to improve the fuel efficiency of their cars and trucks.  Most recently, the emphasis has been on reducing weight by changing to less dense materials even though they’re more expensive.   There is also ongoing development work with electrification to recover and reuse kinetic energy.  The latest change to help manufacturers comply with tightening fuel economy targets worldwide is revamping the automatic transmission.

Historically, automatic transmissions were inherently less efficient than manual gearboxes, and convenience was the tradeoff for the loss of a few percentage points in fuel economy and acceleration. Some of the latest automatic gearboxes, though, are actually more efficient than a manual gearbox with a clutch.  Today, the desire to retain complete manual control over gear selection means, in some cases, slightly higher fuel consumption and longer 0 to 60 mph times.  However, a stick-shift generally still saves money off the new sticker price and in North America is sometimes regarded as an anti-theft device.

On Up to 10

From the late 1960s, three speeds was the standard automatic configuration, and it wasn’t until the early 1980s that overdrive and lock-up top gears were added to help improve the efficiency, leading to more four- and five-speed automatic gearboxes.  In 2002, gearbox technology began to get a lot more attention when BMW put the first 6-speed automatic into production, followed by Mercedes with its 7-speed in 2003 and Toyota with an 8-speed in 2007.  Recently there have been a number of transmission announcements:

  • GM is crediting its new 8-speed automatic for making the 2015 Corvette Stingray faster and more efficient.  More gears allows for a lower first gear ratio for better acceleration, as well as a higher final drive ratio to reduce engine speed at highway cruising speed.  The 8L90 transmission will also feature in GM’s range of pickup trucks and SUVs.  Careful packaging and internal design features means that the new gearbox fits the same space as the 6-speed 6L80 – even though it can handle higher torque and power in addition to weighing less.
  • ZF introduced its revised 8-speed transmission in the 2014 BMW 5 Series.  This second-generation 8HP gearbox (the first was introduced in 2009) offers revised gear ratios to take advantage of the latest engine efficiency improvements that deliver more torque at lower rpms.  Advanced torsional vibration dampers improve smoothness, and a new shifting design has reduced internal energy losses.  Other users of the 8HP for rear-wheel-drive cars are Audi, Jaguar Land Rover, and Chrysler.
  • Chrysler is building a 9-speed transmission under license from ZF.  It went into production at the end of 2013 in the Jeep Cherokee.  ZF also supplies the 9HP for the Range Rover Evoque.  Chrysler is planning to implement a version in its minivans and smaller front-wheel-drive cars, as well.  Although the wider ratios provide better fuel economy and acceleration, concerns have been raised about erratic shifting.  These are being addressed via a software update.
  • In September 2013, Mercedes launched its 9G-TRONIC transmission on the E 350 BlueTEC diesel saloon car.  Despite two additional gears and a higher maximum torque, the new automatic transmission requires no more installation space than its predecessor and is also lighter.  The torque converter housing is made of lightweight aluminum, while the transmission housing with plastic oil pan is made of an even lighter magnesium alloy.

Ford and GM have already announced that they are planning to develop a 10-speed gearbox together.  It seems that manufacturers have figured out how to get more ratios in the same space and, at the same time, reduce internal energy losses so that efficiency is higher while maintaining or improving performance.  The key is integration with the latest engine characteristics to optimize the driving experience.  This topic will be covered in more detail in our upcoming report on automotive fuel efficiency.

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