Attempts to improve fuel efficiency of farm tractors motivated the development and application of viscous fan drives to agricultural tractors. Early drives on tractors performed in an on-off fashion similar to drives widely used in on-highway trucks and passenger cars. Due to the high engine power and slow moving operational modes of farm equipment, a drive which continuously modulated output speed as a function of cooling requirements was desired. A design was generated and applied to agricultural tractors. Resulting vehicle performance improvements include reduced fuel consumption, reduced operational noise, and reduced engine warm-up time.