I drive a 2011 BMW 323i Steptronic. I know cars fitted with automatic transmissions are generally heavier on fuel than their manual counterparts. Would I save fuel if I changed gear manually instead of leaving the gearbox in full-auto mode?
NICKULAAS SCHUTTE, Germiston
Answer: No. The reason older vehicles equipped with automatic transmissions use more fuel than manual versions is due to the slippage in the fluid coupling (the torque converter). The wasted energy leads to the build-up of heat in the automatic-transmission fluid. Most modern automatics that still employ the torque-converter type now have a lock-up clutch to negate the slip once a gear is engaged. Overriding the auto function in your BMW’s transmission doesn’t do away with the converter, so the losses remain. Therefore, it’s best to leave it in auto mode because the shift points are calculated to occur at optimal times when driving in a sedate manner.
Many autos now have up to eight (or even nine) gear ratios, which allow the engine to operate at the most fuel-efficient speeds, allowing them to eclipse the fuel consumption of a six-speed manual transmission. The dual-clutch transmission type does not use a torque converter, so it is as efficient (or even more so) than a manual.