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Technology June 2012

Transmissions -- More Gears Are Better

By Bill Siuru

Besides achieving better fuel economy, engines don't have to work as hard so they will last longer and require less maintenance because there is less stress and wear. When engines run at lower rpms at high cruising speeds they are quieter.

Remember when cars came with three-speed manual transmissions and automatics had only two or three gear ratios? Back then if you wanted maximum fuel economy, you opted for "three on a tree," a three-speed manual with the gear selector attached to the steering column. Automatic transmissions, often called "slush boxes" because of their poorer efficiency, did make driving easier, but got noticeably fewer miles per gallon.

How things have changed. Today, five-speed manuals are the norm and some high performance cars have six speeds. Automatics have at least four – or as many as ten – speeds. Eight-speeds are already available in several BMW, Audi, Lexus, and Chrysler models. A nine-speed could first be used in a upcoming Chrysler model. Hyundai is now developing a ten-speed automatic.

Along with advanced engines and slick aerodynamics, high-tech transmissions are also playing a big role in achieving better fuel economy. Engines produce the greatest horsepower and torque while consuming the least amount of fuel when running in a relatively narrow engine speed – that is rpm (revolutions-per-minute) – band. Having more gear ratios in a transmission results in a greater ability to operate an engine within this optimum, narrow speed range. That is why big trucks have up to 18-speed transmissions.

How much does adding more gear ratios increase mpgs? Transmission manufacturer ZF says an eight-speed automatic transmission can improve fuel economy 21 to 24 percent over a three-speed automatic. This is in addition to mpg increases resulting from improvements in engine design. An eight-speed provides an 11-percent saving compared to a six-speed transmission and 14-percent versus a five-speed. Five- and six-speed transmissions are common in today's cars, SUVs and light trucks.

Gone are the days that when you wanted the most mpgs, you ordered a manual transmission. Today, EPA highway and city mileage ratings for many vehicles are identical whether equipped with an automatic or manual transmission. Automatics can even do better in many instances. Computerized, electronically-controlled transmissions can shift through the gears more effectively than all but most accomplished driver and do it with imperceptible gear changes.

Besides achieving better fuel economy, engines don't have to work as hard so they will last longer and require less maintenance because there is less stress and wear. When engines run at lower rpms at high cruising speeds they are quieter.

Could we see ever more gear ratios in transmissions? Some experts say we may be reaching a point of diminishing returns because internal friction and energy losses can cancel out improvements made in efficiency.

However, cars, especially hybrids, are already using continuously variable transmission, or CVTs, that have essentially an infinite number of gear ratios. These really are the modern version of the shiftless transmission used in the Cushman motorscooter you might have owned as a kid. Because of current torque transmission limits, the use of CVTs has been limited pretty much to vehicles with relatively low power engines.

Finally, electric vehicles can get by quite nicely with a single-speed transmission. This is because electric motors produce the same constant peak torque from zero to maximum rpm. This also gives EVs outstanding low speed acceleration.

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