If the wrong mileage rate is used for expense claims then this is most likely because the vehicle is associated with the wrong rate. Once the correct one has been established, it will be necessary to create a new vehicle to associate with it - flagging the original as No longer in use if appropriate.
Alternatively, it may be that the claimant has reached a rate change threshold, and the rate is calculating based on some miles at one, and the rest at another. This might be particularly evident if the rate being shown does not exist for any other mileage type.