This article analyzes the revolutionary impact the tractor had on rural America and examines the economic, technological, and institutional factors governing the machine's diffusion. Our simultaneous-equation regression model helps decipher the complex relationship between farm scale and diffusion. In addition, analyzing diffusion as a capital replacement problem reveals that the shift to the new technology was far more rapid than what has generally been thought and provides a new perspective on the long co-existence of both horses and tractors on farms.