Gearing Up: Why the Humble Transmission Is Now the Hottest Tech Under the Hood

Last Updated May 11, 2011 6:11 PM EDT

A current GM six-speed transmission: outmoded technology?
You probably haven't noticed, but automakers are adding extra gears to both manual and automatic transmissions with abandon in order to eke out additional fuel economy. That begs an obvious question as General Motors (GM) and Chrysler retool to build eight-speed transmissions: Is there a point of diminishing returns where it actually becomes counterproductive to add another gear?

The consensus among automakers is that there is such a point for today's engines, and with eight speeds we're starting to get close to it. No doubt, though, that we'll see some trannies with double-digit gearing, squeezing out increasingly small fuel economy advantages and trying for that magical 40 mpg on the highway.

It's all about fuel economy
Automakers aren't doing this for bragging rights. Most Americans drive automatics, and have no idea how many speeds are inside their transmissions -- and there's no reason for them to care. But the MPG window sticker matters a lot these days, so automakers have focused on a particularly ripe piece of low-hanging fruit. Federal regulation that could require cars to reach 60 mpg combined by 2025 is also a big part of the picture.

When automakers didn't worry about fuel economy (because consumers didn't), they let transmission tech remain static. American cars went for decades with lazy two-speed automatics and three-speed manuals. But General Motors said Monday that it's investing $260 million in its Toledo, Ohio plant to make eight-speed automatics, and other companies are following suit. Chrysler is waiting for a tax abatement to announce a similar $843 million retooling at its plant in Kokomo, Indiana.

Inside your transmission
A tiny bit of transmission tech: Engines are at their peak efficiency at only one spot in each gear, so adding more gears creates more opportunities to max out the power plant. Moving from four speeds to six on automatics added a not-inconsiderable four to six percent fuel economy gain. One result of all the improvements is that manual transmissions no longer have their traditional three to five mpg benefit, and sometimes today's automatics have more gears than manuals.

If adding gears made trannies much heavier and bulkier, the bottom line would turn negative pretty quickly. But since auto companies are finally devoting resources to what was once a technical ghetto, design improvements have created multi-gear transmissions without any significant packaging sacrifice. But it won't last forever.

No upper limit, but 12 speeds is unlikely soon
Automakers won't be tied to an upper gear limit, but they say some sort of natural limit may be ahead -- though a technical breakthrough would mean all bets are off. Mircea Gradu, director of transmission and driveline engineering at Chrysler, sees a plateau ahead:

I can't say eight speeds is the limit, absolutely not -- we will evolve to a higher number of gears. But with the power plants we have now there is a leveling off of the benefits after a certain point.
Jim Lanzon, GM's executive director of transmission engineering, mostly agrees:
Plain physics dictates a diminishing return, because you start to consume a lot of energy just turning these more complex transmissions over. There are cost considerations, and losses when you add more mechanical junctions. So I don't see 12-speed transmissions on the immediate horizon. But we're always looking for the next new transmission innovation, and if it emerges we'll be all over it. Thirty years ago, I would have said we'd never see six speeds.
GM spent more than $800 million since 2006 to equip Toledo to produce six-speed transmissions (now used on everything from the Cadillac Escalade to the Chevrolet Cruze and Malibu). But the state-of-the-art is moving fast, so six speeds are no longer good enough.

The latest transmission plant retrofit (part of a larger $2 billion upgrade of 17 plants announced in Toledo) represents thinking ahead, something GM hasn't always done. The company doesn't even currently offer a car with an eight-speed transmission, though the hybrid Enclave it showed off at the Shanghai auto show uses one.

GM's Tom Read says the company hasn't announced when Toledo will start pumping out those eight-speed boxes, or what cars they might go on. That will all get sorted out. What matters is that GM won't be left behind as other automakers scoop up tranny-related fuel economy benefits.

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Photo: General Motors