[c#] When should use Readonly and Get only properties

In a .NET application when should I use "ReadOnly" properties and when should I use just "Get". What is the difference between these two.

private readonly double Fuel= 0;

public double FuelConsumption
{
    get
    {
        return Fuel;
    }
}        

or

private double Fuel= 0;

public double FuelConsumption
{
     get
     {
          return Fuel;
     }
}

As of C# 6 you can declare and initialise a 'read-only auto-property' in one line:

double FuelConsumption { get; } = 2;

You can set the value from the constructor but not other methods.

readonly properties are used to create a fail-safe code. i really like the Encapsulation posts series of Mark Seemann about properties and backing fields:

http://blog.ploeh.dk/2011/05/24/PokayokeDesignFromSmellToFragrance.aspx

taken from Mark's example:

public class Fragrance : IFragrance
{
    private readonly string name;

    public Fragrance(string name)
    {
        if (name == null)
        {
            throw new ArgumentNullException("name");
        }

        this.name = name;
    }

    public string Spread()
    {
        return this.name;
    }
}

in this example you use the readonly name field to make sure the class invariant is always valid. in this case the class composer wanted to make sure the name field is set only once (immutable) and is always present.

A property that has only a getter is said to be readonly. Cause no setter is provided, to change the value of the property (from outside).

C# has has a keyword readonly, that can be used on fields (not properties). A field that is marked as "readonly", can only be set once during the construction of an object (in the constructor).

private string _name = "Foo"; // field for property Name;
private bool _enabled = false; // field for property Enabled;

public string Name{ // This is a readonly property.
  get {
    return _name;  
  }
}

public bool Enabled{ // This is a read- and writeable property.
  get{
    return _enabled;
  }
  set{
    _enabled = value;
  }
} 

Methods suggest something has to happen to return the value, properties suggest that the value is already there. This is a rule of thumb, sometimes you might want a property that does a little work (i.e. Count), but generally it's a useful way to decide.