1. A diagnostic system for a vehicle that is driven by an internal combustion engine, comprising: a driving pattern acquisition unit that acquires a diagnostic driving pattern defined by a vehicle speed; a drive train model storage unit that stores a drive train model for calculating a torque and rotational speed of the internal combustion engine backward from a vehicle speed and an acceleration; a map data storage unit that stores map data in which a fuel consumption is associated with a torque and rotational speed of the internal combustion engine; a map data updating unit that updates the map data on the basis of a result of actual control over the internal combustion engine; and a fuel efficiency calculation unit that uses the drive train model to calculate changes in torque and rotational speed of the internal combustion engine when the vehicle is caused to drive in accordance with the driving pattern, and that uses the calculated changes in torque and rotational speed of the internal combustion engine and the map data to calculate a fuel efficiency when the vehicle is caused to drive in accordance with the driving pattern.