This paper presents a software process improvement (SPI) initiative conducted at two automotive companies, focusing on the inter-departmental interplay between manufacturing and product development, which are central players in automotive development. In such a complex environment with multiple departments with varying challenges⎯the planning of improvement possibilities was considered as mission critical to get support for changes in the companies. This paper reports the results of the SPI efforts following the process assessment, namely specifically the improvement planning step, which is often overlooked in empirical reports. We also thoroughly describe and report on lessons learned from employing our tailored planning method involving 41 professionals. We found that requirements engineering, early manufacturing involvement and roles and responsibilities were prioritized as main challenges to address. Furthermore, our and the involved professionals' experiences of the used SPI (planning) method, showed that it was useful, giving valuable decision support for the planning of the improvement.