Carpooling has long held the promise of reducing gas consumption by decreasing mileage to deliver co-riders. Although ad hoc carpools already exist in the real world through private arrangements, little research on the topic has been done. In this paper, we present the first systematic work to design, implement, and evaluate a carpool service, called coRide, in a large-scale taxicab network intended to reduce total mileage for less gas consumption. Our coRide system consists of three components, a dispatching cloud server, passenger clients, and an onboard customized device, called TaxiBox. To improve coRide’s efficiency in mileage reduction, we formulate a NP-hard route calculation problem under different practical constraints. We then provide (i) an optimal algorithm using Linear Programming, (ii) a 2 approximation algorithm with a polynomial complexity, and (iii) its corresponding online version. We evaluate coRide with more than 14,000 taxicabs, and the results show that compared with the ground truth, our service reduces 33% of total mileage. This work is published in ACM SenSys 2013.