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A Shift to High MPG

by Bill SiuruJune 9, 2012
Advanced powerplants represent but one method of gaining better fuel efficiency. In the propulsion realm, high-tech transmissions also play an important role. Their evolution is notable. In earlier years, cars came with three-speed manual transmissions while automatics had only two or three gear ratios. Today, five-speed manuals are the rule with some high performance cars […]

Advanced powerplants represent but one method of gaining better fuel efficiency. In the propulsion realm, high-tech transmissions also play an important role. Their evolution is notable. In earlier years, cars came with three-speed manual transmissions while automatics had only two or three gear ratios. Today, five-speed manuals are the rule with some high performance cars upping the ante to six. Automatics have at least four or as many as nine speeds. Hyundai is developing a 10-speed automatic.

Gasoline and diesel engines produce the greatest horsepower and torque, plus use the least fuel, while operating over a relatively narrow rpm range. The more gear ratios in a transmission, the greater the ability to operate an engine within this optimum rpm range.  Eighteen-wheel big rig trucks have as many as 18 speeds in their transmissions for this reason. Incidentally, since electric motors produce peak torque (but not peak horsepower) essentially from zero to maximum rpm, electric vehicles get by quite nicely with a single speed transmission.

Transmission manufacturer ZF calculates that an eight-speed automatic transmission by itself can bring a 21 to 24 percent improvement in fuel economy over a three-speed automatic. ZF eight-speeds are already available in several Audi, BMW, Chrysler, and Lexus products. A ZF eight-speed provides an 11 percent fuel savings compared to a ZF six-speed transmission, and a 14 percent improvement versus a five-speed transmission. ZF has also developed a nine-speed transmission that could first be used in a Chrysler model, bringing with it up to a 16 percent improvement over a six-speed.

An added benefit is that with more ratios available, engines don’t work as hard and thus experience less stress and wear. This means they’re likely to last longer and require less maintenance. Engines running at lower rpms at high cruising speeds are also quieter. Today's multi-speed automatic transmissions make imperceptible gear changes even under high loads.

Multi-speed transmissions are no longer burdened with the fuel economy and performance compromises once inherent with older automatics. Today, EPA mileage ratings for many vehicles are identical when equipped with automatic and manual transmissions, and in some cases automatics are better. This is in stark contrast to just a few years ago when there was a wide difference, and the reason why people seeking the greatest efficiency often opted for manual transmissions.

The success of today's multi-speed automatics can largely be attributed to advanced electronics and on-board computers. They can shift through the gears more effectively than all but the most accomplished drivers, providing efficient gear transitions ranging from performance crisp to luxury smooth depending on vehicle type and the target customer.

The importance of these advanced transmissions will only grow as discerning buyers select their new car purchases in a competitive market where fuel efficiency continues to be considered a buying imperative.