A Little Knowledge of Transmissions Can Save You Big Bucks at the Pump

By Scott Doggett July 12, 2008



Choosing a fuel-efficient automobile was a whole lot simpler 30 years ago, the last time the U.S. was in the throes of a gasoline price crisis. It was often as simple as buying a small vehicle with a manual transmission.

Today, with gas prices soaring to new record highs almost weekly, finding fuel economy means navigating an almost bewildering selection of vehicles. Even enormous SUVs can qualify if they come equipped with gasoline-electric hybrid drive systems.

Behind this tremendous hike in choices -- to a fairly significant extent, experts say -- has been a concomitant increase in the variety of transmissions.
Whereas in the 1970s there were only four- and five-speed manual transmissions and three- and four-speed automatic transmissions, now there are six-speed manual, six-, seven- and eight-speed automatic, six- and seven-speed "dual-clutch" automatic-manual, and continuously variable transmissions.
The result: more efficient engine operation and higher fuel economy in every car, whether it's a high-performance Porsche Carrera or a modest Volkswagen Jetta.

But there are important differences among these sophisticated transmissions, including in how much they contribute to better fuel efficiency. Knowing those differences may help you the next time you're in the market for a new car.

The whole point of the transmission is to allow the vehicle's engine to operate at its most efficient state under all conditions and demands.

Gear ranges help do that by matching the engine's speed, or RPM (revolutions per minute) and torque - or pulling power - to the demands of acceleration. Lower gears provide less speed but more torque to get the vehicle's mass in motion, whereas higher gears provide more speed, but less torque, to keep things rolling along.
The engine is at peak efficiency at only one point as its RPMs climb through each gear range, so as more gears or "speeds" are added to a transmission, the more high-efficiency spots are available and the more efficient the engine's overall operation.

Transmission specialists at General Motors, Ford and Chrysler say 6-speed automatic transmissions will yield a four to six percent fuel economy improvement compared to 4- and 5-speed automatics.
A 6-Speed Revolution

Each of the companies offers 6-speed automatics in some vehicles now, and both GM and Ford have said they will adopt the technology even more broadly in the near future.
GM launched its first 6-speed automatic in 2006 and since then has developed nine variants and offers the transmissions in 40 vehicles. A tenth variant is due to be released in 2009, and by 2010 the company predicts it will be producing three million 6-speed transmissions annually.

Ford says that by the end of 2009 it will more than double - to 1.4 million - the number of North American cars and trucks it equips with 6-speed automatics. And the company pledges that 98 percent of its North American automatic transmissions will be 6-speeds by 2012.

The extent of the mileage boost from additional gears will vary from vehicle to vehicle - affected by a wide range of factors, including the engine's characteristics, the kind of vehicle in which it's installed, and even the method for testing fuel economy.
Moreover, the boost isn't always in the same proportion as gears are added. An 8-speed automatic transmission may offer as little as two percent or as much as seven percent better fuel economy than a 6-speed automatic, says Hamid Vahabzadeh, director of powertrain advanced automatic transmission engineering at GM in Pontiac, Mich.

Diminishing Returns
There is a point of diminishing returns as the number of gears increases, says Mircea Gradu, director for transmission and driveline engineering at Chrysler in Auburn Hills, Mich. But it is still possible to justify a 7-speed or 8-speed automatic transmission.
From an engineering standpoint, he says, an 8-speed automatic can be a more elegant solution than a 7-speed automatic - more efficient and more compact with fewer components. Chrysler is considering using both 7-speed and 8-speed automatic transmissions in future vehicles, he adds.
An 8-speed can be more compact than a 7-speed because, with the newest ways to build automatic transmissions, it is not necessary to increase the number of gear sets to match the transmission's "speeds," says Paul Williamson, national manager of Toyota's Lexus College.
For this reason, he says, the new 8-speed automatic offered in Lexus cars is no larger than the company's previous 6-speed automatic, and its new 6-speed automatic (used in Lexus, Toyota and Scion vehicles) is no larger than its previous 5-speed manual or 4-speed automatic transmissions.

Fuel-Efficient Automatics
Now, too, "automatic transmissions no longer always get worse fuel economy than manual transmissions," and in some vehicles the two get identical mileages, says Williamson, whose Toyota Motors USA unit, in Torrance, Calif., trains all U.S. Toyota dealers on the inner workings of the company's vehicles.
He said it's no longer uncommon for automatics to have more gear ratios than manuals. These are major changes from the 1970s, when manual transmissions typically delivered three to five more miles per gallon fuel economy than automatic transmissions, which almost always had fewer gear ratios than manual transmissions, he adds.
Today, the BMW 328i coupe or sedan with a 6-speed automatic transmission is EPA rated at 19 miles per gallon in city driving and 28 mpg on the highway, while the same car with a 6-speed manual transmission is rated at 18 mpg city and 28 mpg highway.
The higher-performance BMW 335i is rated at 17 mpg city and 26 mpg highway with either a manual or an automatic transmission.
As automakers struggle to keep up with consumer and government demands for ever-better gas mileage, "we're going to have to look for gains everywhere we can find them, and that includes the transmissions," declares Dave
Buchko, a BMW spokesman.
BMW, like many automakers, programs its automatic transmissions to keep the torque converter - a source of energy loss that can be a drag on fuel economy - "locked up" as often as possible to help boost the transmission's overall efficiency.
Advanced electronic controls that regulate communications between the engine and the transmission also are playing an important role in boosting fuel economy, Buchko says, by keeping the transmission in the higher gear ranges for longer periods.

More Than The Gears
The broad issue, says Williamson, is the efficiency of the vehicle's entire drivetrain - all of the parts that transmit power to the wheels. Thus, for example, Mercedes-Benz has re-engineered its 4MATIC all-wheel-drive system in the 2009 CL550 and 2008 C300 and S550 to integrate the transfer case into the cars' 7-speed automatic transmission, reducing the system's size and weight.
As a result, notes Mercedes-Benz spokesman Robert Moran, the 4MATIC C300 and S550 achieve the same fuel economy as their two-wheel-drive counterparts - 17/25 for the C300 and 14/20 for the S550. (The rear-wheel-drive S550 is rated at 14/21.)

"It's not always the number of gears, it's the entire system," concurs Paul Olexa, vice president of driveline sales for North America at ZF Group in Northville, Mich.
ZF, a leading European transmission maker based in Friedrichshafen, Germany, was the first to introduce a 6-speed automatic transmission for passenger cars, in 2001 in the BMW 7 Series sedans.
It is now developing an 8-speed automatic transmission that is expected to launch in a commercial vehicle late next year - and which debuted at the 2008 Geneva auto show in BMW's X5 Vision EfficientDynamics, a diesel-electric "mild" hybrid SUV concept.
CVT or DCT?

Some transmissions do away with traditional fixed, toothed gears in the hunt for optimum efficiency. A continuously variable transmission, or CVT, is composed of pulleys connected by a belt.
The pulley on the transmission input shaft -- which transmits the power coming from the engine, has sides that that move apart or draw together to form a groove in which rides a V-belt connected to the drive shaft. As they get closer together the belt rides higher, with a larger radius, and as they move apart the belt rides lower in the groove, shrinking its radius.
The radius changes alter the speed at which a fixed pulley at the other end of the belt rotates the drive shaft, effectively substituting for fixed gears.
Because the width of the groove can be infinitely adjusted, it creates an infinite number of discrete gear ratios in very fine increments - allowing the engine to always operate at optimum efficiency.

All this, of course is controlled by sophisticated electronics.

The complete story as seen/read on edmunds.
Source - edmunds Auto Observer: http://www.autoobserver.com//2008/07...-the-pump.html