A farm modeling package called the Common Agricultural Tractor Selection System (CATSS) has been created to generate a recommendation for the most cost-effective size and type of tractor for spring tillage/planting operations. The CA TSS model combines mathematical models of numerous aspects of a farming operation. Climate conditions are simulated using a stochastic method based on historical data. The average R2 for the seven simulated climatic elements is 0.72, although some elements reach an R2 > 0.97. The simulated climatic variables are used to calculate other weather elements, such as topsoil temperature and dew-point temperature. The climatic data is combined with soil type and past tillage practices to predict the soil conditions. Two soil hydrologic models are available for use: the Saxton and Rawls - Texture and Organic Matter (SR-TeOM) model (2006), and the Soil Texture and Compaction (STAC) model (Pulley, Min, and Chaplin 2008). The climatic and soil characteristics are then used with a simple hydrologic model to estimate the soil moisture content. A linear model is used to estimate the soil strength, quantified by the cone index. The soil moisture content (normalized across soil types) and the level of soil compaction are the independent variables. Using this estimate of the cone index, Wismer-Luth tractive theory (1974) can be applied. The tractive performance in the simulated field conditions is used to optimize the tractor and implement combination for a farm operation. Lastly, economic principles are used to calculate the component costs for each tractor size/type, and find the least-cost combination.