In this paper, we propose a novel energy-efficient data transmission reduction method for periodical data gathering in Wireless Sensor Networks (WSNs). Our method exploits overhearing, which is a typical characteristic of wireless communications. In our method, each node in a WSN autonomously determines whether its own reading is redundant or not by using the overheard packets transmitted by its neighbors. The redundancy of a reading is evaluated by a simple spatial interpolation using the readings of its neighbors. If the reading is determined as redundant, the node stops transmitting it. Furthermore, we present an evaluation based on simulation experiments with various parameters and show that our method suppresses data transmissions and reduces total energy consumption.