<xctk:DoubleUpDownFormatString="F2" Increment="0.01" Minimum="0" Value="{Binding Main.Speed}"></xctk:DoubleUpDown>

private double _speed;
public  double  Speed
{
    get
    { return _speed;  }
    set
    {
        _speed = value;
        RaisePropertyChanged("Speed");
    }
}