Transmission Gear Ratio Spread

How far to go?

Emission and fuel efficiency norms of today are demanding cleaner and more efficient vehicles. OEMs and suppliers have responded to this by introducing down-sizing with use of a turbocharger for both diesel and gasoline engines. Powertrain design is incomplete without considering transmission design to match with the engine for optimum fuel savings and performance. Manufacturers have been developing new transmissions to incorporate the change in input power and deliver increased torque distribution with less fuel burn by offering more transmission ratios. This enables smaller engines to operate much nearer to their optimal acceleration, speed and torque. Two decades ago, a standard manual gearbox had only three to five speeds. Today at the end of 2018, we have automatic transmissions up to 10 speeds. In 2017, Ford and GM collaborated to develop advanced 10-speed transmissions (10R60/10R80) installed in RWD/AWD vehicles such as the 2017 Ford F-150, 2018 Expedition, 2018 Lincoln Navigator, 2017 Chevrolet Camaro ZL1, 2018 Yukon Denali and 2018 Tahoe RST. GM also designed a 9-speed transmission (9T45/9T50) in 2017 for FWD vehicles, which include the 2017 Chevrolet Malibu (gasoline) and 2017 Chevrolet Cruze (diesel). ZF’s 9HP 9-speed transmission is also popular among FCA’s offerings, such as the 2017 Chrysler Pacifica and 2018 Jeep Compass. The question raised in this analytical piece is how many more gears are necessary and what portion of power could be compromised to attain even higher efficiency from higher-geared transmissions. Some OEMs like Honda have gone further ahead and designed production-ready versions of 11-speed transmissions with triple clutch along with a 10-speed DCT for future Honda City (Fit) models.