With the development of autonomous cars, the demand for accurate light detection and ranging (LiDAR) systems is increasing. Previous evaluations of LiDAR were mainly based on experiments and lacked theoretical significance. We theoretically evaluate the accuracy and optical output power of LiDAR systems in autonomous cars. We focus on two ranging schemes: the time-of-flight (TOF) method and the quadrature phase detection (QPD) method. Considering the special requirements of autonomous driving, the theoretical limits of ranging accuracy are calculated by deriving the Cramer-Rao bound (CRB). The influence of reflectivity as well as distance on accuracy are discussed. We also determine the relationship between optical output power and essential parameters, and make comparisons between TOF and QPD. It can be concluded that TOF is more efficient under most circumstances. When designing autonomous cars, such theoretical evaluation provides guidance for choosing laser emitters and receivers, justifying the significance of our work to LiDAR development.