Originally a paid asset, NWH Vehicle Physics was made open-source and is, without question, the most comprehensive car physics solution for Unity on GitHub. It includes:
Car Physics in Unity — GitHub Project Overview car physics unity github
Searching for "car physics unity github" reveals several high-quality open-source projects and frameworks tailored for different simulation needs, from high-fidelity realism to arcade-style handling. Top Open-Source Frameworks on GitHub Originally a paid asset, NWH Vehicle Physics was