A vehicle connected to the Internet of Vehicles (IoV) communicates wirelessly with other vehicles, pedestrians, roadside units and sensors to improve driving comfort, traffic flow and road safety. The IoV poses significant security and privacy threats, posing a risk to human lives. Trust must be established among these potentially untrustworthy vehicles. In addition to scalability and single points of failure, current mechanisms face concerns such as sparsity, availability, consistency, robustness, privacy and efficiency. It is considered a promising solution to establish trust in IoV, because blockchain technology has already proven its success with cryptocurrencies and smart contracts. Decentralized trust management for vehicular networks is proposed in this paper using blockchains. We evaluate its efficiency against malicious vehicles and compare its performance with three hybrid trust models through extensive simulations. Our model achieves over 97% trust level, surpassing existing models that fall below 90% in networks with 25% malicious vehicles.