This paper proposes an autonomous power supply to feed the nodes of a Wireless Sensor Network (WSN) used in vehicles. The vibration levels detected in a moving vehicle in an urban or semi urban area are used as the primary form of autonomous energy. Measurements show that a maximum power density of −16.9 dB/Hz is observed around 15 Hz for cruising speed up to. Our design is based on a cantilever piezoelectric transducer 90 km/h mechanically adjustable in frequency designed and manufactured to resonate very close to 15 Hz. Experimental results clearly demonstrate a level of energy sufficient to adequately supply power to a wireless sensor node. Specifically, for an optimum load resistance of 73.13 kΩ, a power of 3 µW is achieved by our autonomous supply.