With great potential to support multitudinous services and applications in intelligent transportation systems, cognitive satellite-vehicular networks, an emerging paradigm of cognitive satellite-terrestrial networks, are attracting increasing attentions. To realize friendly coexistence of satellite and vehicular networks as well as efficient resource utilization, we investigate power allocation for the cognitive satellite-vehicular network under a realistic 3-D vehicle-to-vehicle channel model. Specifically, by analyzing the characteristics of energy efficiency (EE) and spectral efficiency (SE) performance in different vehicular environments, we first develop an EE–SE tradeoff metric with a preference factor. Based on the developed metric, we formulate and analyze a power allocation strategy from the EE–SE tradeoff perspective while guaranteeing the interference power constraint imposed by satellite communications. Further, utilizing the obtained optimal transmit power, we derive a closed-form expression of the outage probability, through which the impacts of the preference parameter and interference constraints on the performance of vehicular communications can be theoretically analyzed. Finally, numerical results are provided to demonstrate the viability of the EE–SE tradeoff metric and the validity of theoretical analyses.