A fuel economy calculator system for a vehicle including a fuel cell, a diagnostic port, and an ECU, the fuel economy calculator system includes a dongle configured to connect with the diagnostic port to enable the dongle to establish a wired communication link with the ECU, and a wireless device including a display unit, a memory storing program instructions and capacity data of the fuel cell, and a processor. The processor is configured to execute the program instructions to establish a link between the ECU and the wireless device, to receive a data set from the ECU via the connected dongle each time the ECU generates engine change state data, the data set including at least mileage data and remaining fuel data, to calculate fuel economy data of the vehicle based on the capacity data, a most recent data set, and a next most recent data.