High range and speed resolutions provided by FMCW automotive radars are prone to range cell migration in high speed scenarios. The range and Doppler information are smeared through multiple frequency bins in fast an slow-time dimensions, resulting in biased estimations and worsened separation performance. In this paper, the FMCW signal model with inter-pulse range cell migration is derived, the induced bandwidth spreading is illustrated and quantified with simulations. Windowing is shown to mitigate this spreading.