Manual transmissions: losing their fuel economy advantages


Manual transmissions are losing their economy advantage - even in small cars. Manual transmissions have always been more efficient than automatics but the advantage has been significantly reduced in recent years, particularly in the larger car segments. This is because larger, more expensive cars now feature six-, seven- and even eight-speed autos while smaller, less-expensive cars tend to feature four- and five-speed automatics that are less efficient.

Along with more ratios and electronic control, the efficiency of the torque-converters used in automatics, which have been the main culprits in wasting energy, has been dramatically improved and while test programme figures approach the ideal, real-world manual transmission drivers do not always select the most appropriate ratio for the circumstances, which the automatic is programmed to achieve at all times.

The narrowed gap in the large car segment is illustrated by the fuel economy of the Holden Commodore SSV, which has six-speed manual and automatic transmission options rated identically at 13 litres per 100km (l/100km) according to www.rightcar.govt.nz. However, in the mid-size segment, a 2.0-litre Mitsubishi Lancer Evolution X with a six-speed auto is rated at 10.6l/100km, 4% down on the five-speed manual’s 10.2l/100km. In small cars, however, the advantages of the manual become more apparent. For example, the Suzuki Alto five-speed manual is rated at 4.8l/100km - around 13% better than the four-speed auto’s 5.5l/100km. Similarly, the Honda Jazz five-speed manual option is rated at 5.7l/100km, nearly 14% better than the four-speed auto’s 6.6l/100km.

Significantly though, the Jazz is also available (except the current model sold new in NZ) with a continuously-variable transmission (CVT), a technology being used more and more and enhances fuel economy by operating the engine in its most efficient range. The CVT Jazz is rated at 5.8l/100km – a mere 2% down on the manual and with the convenience of an automatic although the same comparison with a 2.0-litre Mitsubishi Lancer reveals that the CVT model is nearly 8% down on the five-speed manual at 8.2l/100km versus the latter’s 7.6l/100km.

Nissan put its faith in CVT technology some years ago and has improved its torque-handling capability to the degree that it is now a popular option on the company’s 3.5-litre vehicles. On the Maxima, for example, there is no longer a manual option available because the CVT model achieves 10.2l/100km - 5.6% better than the six-speed conventional auto’s 10.8l/100km.

Meanwhile, manual transmission technology has not been standing still. Many automotive manufacturers and suppliers have developed automated manuals over the years but shift quality was typically poor prior to the development of the dual-clutch transmission (DCT), which Volkswagen first launched into the market as the Direct Shift Gearbox (DSG). DSG shift quality is almost seamless and the fuel economy losses from the energy required to operate the clutches and gear selectors reduce the fuel economy minimally to 7.4l/100km from the manual’s 7.3l/100km in the Golf GTI. However, those losses can often be retrieved by the transmission outperforming a human driver by always selecting the most appropriate ratio for the conditions.

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