If you operate on the base type, you can only access public methods and fields of it.
If you want to access the extended type, but have a field of the base type where it's stored (as in your case), you first have to cast it and then you can access it:
Car car = (Car)myGarage[0];
car.doSomeCarStuff();
Or shorter without temp field:
((Car)myGarage[0]).doSomeCarStuff();
Since you are using Vehicle objects, you can only call methods from the base class on them without casting. So for your garage it may be advisable to distinguish the objects in different arrays - or better lists - an array is often not a good idea, since it's far less flexible in handling than a Collection-based class.
