How to serialize a class that contains properties of type interface (contract design)

Module Module1
 
    Sub Main()
 
        Dim bmw As New Car
        bmw.Tyres = New List(Of ITyre)
 
        Dim t As ITyre
 
        t = New BigTyre
        t.Size = 14
        bmw.Tyres.Add(t)
        t = New BigTyre
        t.Size = 15
        bmw.Tyres.Add(t)
 
        Dim mem As New IO.MemoryStream
        Console.WriteLine("Here comes an exception that says, that tyre is not serializable because it is an interface")
        Dim ser As New Xml.Serialization.XmlSerializer(GetType(Car))
        ser.Serialize(mem, bmw)
 
        bmw = Nothing
        mem.Position = 0
 
        bmw = ser.Deserialize(mem)
 
        Console.WriteLine("The number of tyres should be 2")
        Console.WriteLine(bmw.Tyres.Count)
 
        Console.ReadLine()
 
    End Sub
 
End Module
 
Public Class Car
 
    Private _tyres As List(Of ITyre)
    Public Property Tyres() As List(Of ITyre)
        Get
            Return _tyres
        End Get
        Set(ByVal value As List(Of ITyre))
            _tyres = value
        End Set
    End Property
 
End Class
 
Public Class BigTyre
    Implements ITyre
 
    Private _size As Double
    Public Property Size() As Double Implements ITyre.Size
        Get
            Return _size
        End Get
        Set(ByVal value As Double)
            _size = value
        End Set
    End Property
 
End Class
 
Public Interface ITyre
 
    Property Size() As Double
 
End Interface