Improving Fuel Economy by Increasing Gear Steps of Automatic Transmissions

A method for the determination of the optimum number of gear ratio steps is discussed in consideration of transmission efficiency, vehicle weight, and engine performance, with a view to improving the fuel economy of passenger cars.

Shifting mechanisms which had gear ratios in the practical 5-8 step range with common planetary gear trains were selected. The efficiency of each type was estimated based on the measurement of spin losses on 4-speed automatic transmission units, with considerations of the number of gear trains, clutches etc. The effect of the increase in gear ratio steps on fuel economy was simulated for different combinations of vehicles of various weights and engines of different performance.

The 5-speed unit was found to be the most practical among the transmissions studied in effectively improving fuel economy. It was also found that improvement in fuel economy is greater on cars having higher engine torque per vehicle weight.