Do More Gears Mean Better Fuel Economy?

By Bill Howard

Posted Oct. 7, 2008, at 1 p.m.

Forty years ago, some cars had two-speed automatic or three-speed manual transmissions. Now, it's possible to have as many as eight forward gears, and more gears on automatics than manuals. It's no longer a given that an automatic transmission will be less efficient than a manual. And we're seeing the growth of transmissions that are effectively automated manual transmissions — a manual transmission that shifts itself.

Why More Gears Matter

Torque is what gives an engine its get-up-and-go power and the peak (maximum) torque isn't at the top engine speed but somewhere in the middle, often between 2,000 and 3,500 revolutions per minute on an engine capable of 6,000 RPM. The more gears the car has, the more likely the engine will be operating at its most efficient and most powerful speed. Automakers say that with each added gear, you get 2 percent to 5 percent more economy.

Manual and automatic transmissions typically range from four to six speeds. High performance cars occasionally have seven speeds (Lexus and Mercedes-Benz), and an eight-speed automatic transmission is due to be introduced in 2009 in the Lexus LS 460. If you're buying a car, you won't get to choose a four-speed, five-speed or six-speed automatic. Usually the automaker offers one automatic (no clutch pedal) transmission with a certain number of gears, take it or leave it, and on smaller or sportier cars, a manual transmission with a foot-operated clutch pedal. Even with more gears, automakers have been able to package the transmissions so six- and seven-speeds take up no more room than previous four-and five-speeds. Ford has committed to have the majority of its line offered with six-speed automatics by 2012.

Among these choices, there are variations including:

Continuously variable transmissions (CVTs). A handful of mostly smaller cars use CVTs with infinitely variable ratios. Most use a high-strength belt that runs between two pulleys that can vary their diameters and thus the ratios between the two. (Think of a bicycle with variable size front and back sprockets.) Vehicles with CVTs include the Dodge Caliber, Jeep Compass, Jeep Patriot, Mercedes-Benz Smart Fortwo, Audi A3, and Audi TT. But it has been hard to make CVTs work reliably in heavier, full-size cars.

Double-clutch transmissions (DCTs). A new technology combines the best of manual and automatic transmissions. The double-clutch transmission (sometimes double-clutch gearbox) is a manual gearbox at its core, but there's no clutch pedal. There still is a clutch, the device that connects and disconnects the engine from the drive wheels. (Think of the clutch as two pie plates faced with sandpaper.) The DCT engages and disengages gears automatically for you, or you can shift yourself using the shift lever or paddle shifters.

These transmissions complement (and in some cases improve on) the choices that have been available for years. For example, the first automated manual transmissions offered on a few sporty cars a decade ago (BMW, Maserati, Ferrari) shifted with a jolt, at least until you hit 100 mph, at which point they smoothed out. These were not much help unless you're on the autobahn or racetrack. Enter the DCT, which has not one but two automated clutches, and the ability to offer lightning-fast shifts that are almost as smooth as automatics. The reason for this smoothness is because when you're in first gear using one clutch, second gear has been set in place and ready to go on the second clutch.

By 2009, you will be able to find double-clutch transmissions on models (sometimes just one or two) from Audi (called DSG), BMW (DCT), Dodge, Ferrari, Mitsubishi (Twin-Clutch SST), Nissan, Porsche (PDK), Volkswagen (DSG), and Volvo (PowerShift). Most will be competitively priced with automatic transmissions. At Volkswagen, the six-speed DSG and six-speed automatic transmissions both cost $1,075 over manual-transmission cars. But BMW's DCT on its $57,000 M3 runs $2,700.

Why Buy a DCT?

Shifts will be smoother than with a manual transmission car, a DCT car may be slightly more fuel-efficient than an automatic transmission car, the driver can control gear selection if desired, and at least for now, DCT has the newness factor: You've got one, your neighbor doesn't.

Even if you are interested in the new transmissions, your choices might be limited. In particular, when buying a new car, it's likely you may get ten paint choices but only two transmission choices. If your next car choice offers a six-speed automatic or a CVT or a DCT, you're in good shape. What's most important is the fuel economy you get.

And sometimes old-fashioned works just fine: The 2009 Honda Fit, for instance, offers a five-speed manual or five-speed automatic. But because of the difference in gear ratios, the Fit automatic ($800 extra) actually wins on overall mileage: 31 mpg in the EPA combined rating for the automatic vs. 29 mpg for the manual.

Bio: Bill Howard is an experienced automotive writer and the editor of TechnoRide.com, the car technology site produced by Ziff Davis. Previously, he was PC Magazine's executive editor. Howard is also vice president of the International Motor Press Association.

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