Discuss with your carpool partners how to distribute the costs of your commute fairly. You can do this by taking turns as the carpool driver, or by having the driver charge each rider a fare that you all agree on. Remember that there are more expenses than just gasoline. To be fair, all riders are expected to participate in cost sharing for the gasoline, tolls, parking, oil, service, and wear and tear. For help, refer to this guide from AAA.