Due to the limited energy supplies of nodes, in many applications like wireless sensor networks, energy-efficiency is crucial for extending the lifetime of these networks. We study the routing problem for multihop wireless ad hoc networks based on cooperative transmission. The source node wants to transmit messages to a single destination. Other nodes in the network may operate as relay nodes. In this paper, we propose a cooperative multihop routing for the purpose of power savings, constrained on a required outage probability at the destination. We derive two efficient power allocation schemes, which only depend on the statistics of the channels. It is shown that energy savings of 90% are achievable in line networks with 10 relays for 10-4 outage probability constraint at each receiving node.