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With growing globalization of the economy, to gain a competitive edge in world markets shortening the product development cycle is crucial. Virtual product development is, therefore, being actively pursued in the off-road vehicle industry. To implement this process successfully, the development of comprehensive and realistic computer-aided methods for performance and design evaluation of off-road vehicles is of vital importance. To be useful to the engineer in industry for the development and design of new products, the computer-aided methods should take into account all major vehicle design parameters and pertinent terrain characteristics. They should be based on the understanding of the physical nature and the mechanics of vehicle-terrain interaction. Their capabilities should be substantiated by test data.

Analytical studies reveal that for a four-wheel-drive tractor with rigidly coupled drive axles to achieve the optimum tractive performance under a given operating condition, the theoretical speed (the product of angular speed and free rolling radius) of the front tires must be equal to that of the rear tires, or the theoretical speed ratio must be one. This paper presents tractive performance test data obtained using an instrumented four-wheel-drive tractor with seven different sets of tires at various theoretical speed ratios. Field data confirm the analytical findings that when the theoretical speed ratio is equal to one, the slip efficiency and tractive efficiency reach their respective peaks, the fuel efficiency (the ratio of drawbar power to fuel consumed per hour) reaches a maximum, and the overall tractive performance is at an optimum.