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In recent years, the contribution of tires on vehicle fuel economy has been garnering attention. Up until now, rolling resistance coefficient (RRC) has been the standard way of measuring the amount of impact the tire has on fuel economy. We devised a new method for evaluating the impact of tires on fuel economy that incorporates the concept of tire “driving transmission efficiency” (hereinafter referred to as “driving stiffness”). In doing so, we have clarified the technology direction for contributing to the improvement of fuel economy while maintaining vehicle maneuverability by reducing RRC and improving tire driving stiffness.

Improving the automobile fuel economy, it is necessary to measure the power loss of the drive train with high accuracy. The authors propose a new method for measuring the friction loss of automobile drive train. This method is based on the measurement of temperature rise of lubricant due to the friction in a drive train, and allows us to separate the friction loss into the lubricant churning loss and the gear friction loss. Applying the new method to a manual transmission and a rear axle, the friction loss of automobile drive train was measured with much higher accuracy, and effects of the gear ratio, input shaft speed, lubricant temperature, kinematic viscosity and differences of drive train on the friction loss have been revealed.