Due to its potential for cutting down energy consumption, sharing transport costs, and reducing negative impacts on the environment, ridesharing has been adopted as a viable model by many cities all over the world to achieve sustainability goals. Although there have been many studies on ridesharing systems, most of these studies have focused on the optimization of performance directly linked to saving energy consumption costs or travel distance. The methods used to divide the cost savings among the ridesharing participants have been less explored. In practice, proportional methods are commonly used to allocate cost savings in ridesharing systems. However, the effectiveness of proportional methods has not been well studied. The goal of this paper is to study and compare three proportional cost savings allocation methods to provide a guideline for choosing an effective method based on an analysis of the properties and performance in terms of the number of acceptable rides and the number of ridesharing participants. The properties were verified by experimental results. This study showed that how cost savings are divided among ridesharing participants has a significant influence on participants’ willingness to adopt the ridesharing transport mode. The properties and experimental results provide a rule and principle that can be used by policy makers and service providers to move towards their sustainability goals by promoting ridesharing through adopting the right proportional cost savings allocation method in ridesharing systems.