Recent decades have witnessed the growing importance of trade in intermediate goods and pursuit of free trade agreements (FTAs). They distort firms’ sourcing decisions internationally through preferential tariffs and rules of origin (RoOs), a set of criteria that define the origin of a product to qualify for preferential access. The paper distinguishes trade diversion through RoOs from tariff reduction on intermediate goods, focusing on the automotive industry. Car assemblers’ decisions of how much to acquire from which supplier are modeled for every auto part. With the derived gravity trade equation, the estimation identifies significant diversion in intermediate sourcing and the effect is nonlinear with respect to the restrictiveness of RoOs. The shift from foreign to regional inputs exhibits a humped shape and it peaks when the required minimum FTA content is between 50% and 60%. Impacts of RoOs are further decomposed to four channels: export destinations of final goods, magnitude of preferential treatments, price and cost penalty of intermediate goods. Results show that the RoO effects are stronger when car exports are mainly intra-FTA. However, there are mixed forces driving the effects of preferential tariff margins of cars and values of parts.