This work has as objective the construction of a reduced fault tree to gearboxes in commercial vehicles. In this reduction, the representative statistical model of reliability is studied for all the components that generate failures, where it is complemented with an application of factorial planning with two replicates to identify the components most sensible to the variation of the parameters in the reliability distributions. Thus, fault tree was constructed from the factors of scale (α) and form (β) for each component. From the data statistical models are obtained to individual failures, and the study of the degree of sensitivity of these components into system becomes viable by the reduction of the model of the FTA and applications of design of experiments. With these techniques, the FTA can be reduced as well as the number of failures around 30%, which contributes in the direction of the improvement of product's reliability, in this case, gearboxes. This procedure can be extended to other systems of a vehicle, allowing the statistics modeling of the system that contribute to the identification of the main components that are the cause or lead to the failures in specific kilometers. This methodology allows to a conception of a model for management of the actions of guarantee and descriptions for the development areas and production. In this model, it is possible to obtain information about time performance that assists in the activities of performance studies and optimization in design engineering. Copyright © 2005 Society of Automotive Engineers, Inc. Blashe, M.K., Shirivastava, B.A., Defining Failure of Manufacturing & Equipment (1994) Proceeding Annual Reliability and Maintainnability Symposium, pp. 69-75Cavalca, K.L., (2000) Apostila de Confiabilidade - IM461 Confiabilidade de Sistemas, p. 110. , Departamento de Projeto Mecânico, FEM, UNICAMPDoty, L.A., Reliability for the Technologies, p. 307. , New York: ASQC Quality Press Book. 1989Klutke, A critical look at the Bathtub Curve (2003) IEEE Transactions on Reliability, 52 (1). , March 2003Marques, D., Metodologia Para Ajuste e Comparação de Modelos de Confiabilidade, p. 2004. , Dissertação de mestrado, Departamento de projeto mecânico, FEM, Unicamp. 125Meeker, W.Q., Escobar, L.A., (1998) Statistical Methods for Reliability Data, p. 680. , John Wiley & Sons, IncMorello, M.G., Silveira, Z.C., Cavalca, K.L., A study of fault tree analysis in mechanical components of automobile industry (2004) SAE: Technical Papers, , São Paulo, SP, BrazilSilveira, Z.C., Cavalca, K.L., Statistical Analysis and Optimization of Design Parameters in Components of Rotating Machinery (2004) Proceedings of the Eighth International Conference on Vibrations in Rotating Machinery, IMechE 2004, pp. 615-626. , Swansea, Wales, UKRamakumar, R., Engineering Reliability: Fundamentals and Applications, p. 482. , Oklahoma: Prentice-Hall International, 1993