The Cellular Vehicle-to-Everything (C-V2X) radio access technology has been specified by the Third Generation Partnership Project (3GPP) in Releases 14 and 15, with special focus on enabling direct communication between vehicles, over the sidelink PC5 interface. More recently, 3GPP has launched the New Radio (NR) standardization activity for the first phase of fifth generation (5G) systems and is ready to enhance C-V2X in several ways under the 5G NR Release 16. 5G NR V2X will encompass flexible numerologies and agile frame structure, higher frequency spectrum, novel and more sophisticated multiple access techniques that well answer the quest for high capacity, ultra-low latency and high reliability of the cooperative automated driving use cases. In this paper, we investigate the impact of the NR flexible numerology, i.e., scalable Transmission Time Interval (TTI) duration and sub-carrier spacing (SCS), on the C-V2X autonomous access mode, according to which vehicles self-allocate resources for transmission. Whereas it is well known that shorter TTI and larger SCS facilitate latency reduction, they have also the potential to mitigate the interference generated by in-band emissions, by better spreading transmissions in the time-frequency domain. Achieved simulation results show that the investigated NR features provide several improvements in terms of message reliability and timeliness when compared to the legacy C-V2X solution.