The repair cost equation is: Ri = w i- W i-1 where Rj = repair cost in year i, Wi = .00000913 C(tD)1 - 5 , C = tractor list price, t = age of machine in years, and D = yearly use in hours. Wjisan altered form of an equation constructed by W.E. Larsen and W. Bowers, "Engineering Analysis of Machinery Costs," presented at the 1965 meeting of the American Society of Agricultural Engineers, Appendix p. 2, June, 1965. 3 The equations for cost components other than repairs were taken from several sources and are summarized in: DarrelD. Kletke, "Dynamic Analysis of Farm Machinery Replacement," unpublished theiis, Oklahoma State Uni- versity, 1968. In this analysis, marginal costs (yearly costs) includes all costs for a given year including overhead.