How to save memory space using JavaScript Prototype [duplicate]

This question already has an answer here:

• Defining methods via prototype vs using this in the constructor - really a performance difference? 3 answers

Take this as an example.

function Car( model, year, miles ) {
    this.model = model;
    this.year = year;
    this.miles = miles;

    this.toString = function() {
      return this.model + "has done " + this.miles + " miles";
    };
}

And we have this:

function Car( model, year, miles ) {
        this.model = model;
        this.year = year;
        this.miles = miles;
}

Car.prototype.toString = function() {
          return this.model + "has done " + this.miles + " miles";
};

And we declare two objects:

var civic = new Car("Honda Civic", 2009, 20000);
var mondeo = new Car("Ford Mondeo", 2010, 5000);

My question is : if we declare function toString inside Car(), or declare function toString outside the Car(), will this make a difference in memory space?

My thought is: the later one, declare function using prototype, outside its constructor, will save memory space, am I right? Why?