When to make a method part of an interface or a concrete implementation?

I create an interface for database access to a MySQL database. Currently, this is the only way to access day, but to make it easier to test and in the event that the data access might change, I used an interface, however, now I am faced with the dilemma of which methods I should put on the interface and which methods I should put on the concrete implementation directly. Is there a rule of thumb for this? I will give an example below where I have some CRUD operations for a ICar interface, but then some other utility functions that are just on the concrete implementation. Should these be on the interface as well? I wouldn't want a user to have to implement every method if they didn't need to?

interface ICar
{
   Car GetCar(int id);
   void DeleteCar(int id);
   void InsertCar(Car car)
   Car UpdateCar(Car car);
}

class Car : ICar
{
Car GetCar(int id)
{

}
void DeleteCar(int id)
{

}

void InsertCar(Car car)
{

}

Car UpdateCar(Car car)
{

}

// methods not on interface, but should they be

public string GetCarVIN(Car car)
{

}

public string GetCarColor(Car car)
{

}

public string GetCarMake(Car car)
{

}

public string GetCarModel(Car car)
{

}

public string GetCarOwner(Car car)
{

}

}