The introduction of LEDs in automotive lighting has enabled disruptive design changes for a range of functionalities. In particular, signal lighting and day-time running lights can now be realized by visually attractive and brand-specific light-guide solutions.Nevertheless, system efficiency has been identified as a key issue in the implementation of light guide solutions. Light-guide efficiency is influenced by three factors. First, the match between far-field pattern given by regulations and generated one. Second, internal losses inside of the light-guide. Finally, the transfer of LED flux into the light-guide input facet.The latter factor, often denoted as incoupling efficiency, is dominated not only by geometrical light-guide dimensions but also by LED shape and radiation characteristics.In this paper, the influence of LED design on the incoupling efficiency is underlined. Comparing dome-shaped and flat-cast LEDs, conclusions from an LED as well as from a chip perspective are derived.