public class CarInfo : DependencyObject
{
    public static readonly DependencyProperty MaxSpeedProperty =
        DependencyProperty.Register("MaxSpeed", typeof (double), typeof (CarInfo), new PropertyMetadata(0.0));

    public double MaxSpeed
    {
        get { return (double) GetValue(MaxSpeedProperty); }
        set { SetValue(MaxSpeedProperty, value); }
    }
}

public class Car : DependencyObject
{

    public static readonly DependencyProperty InfoProperty =
        DependencyProperty.Register("Info", typeof (CarInfo), typeof (Car), new PropertyMetadata(null));

    public CarInfo Info
    {
        get { return (CarInfo) GetValue(InfoProperty); }
        set { SetValue(InfoProperty, value); }
    }

}

<Style TargetType="assembly:Car">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="assembly:Car">
                <Grid >
    !-->            <TextBlock Text="{Binding Path=MaxSpeed}" />
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

<TextBlock Text="{Binding Path=Info.MaxSpeed}" />

<TextBlock Text="{Binding Path=Info.MaxSpeed, RelativeSource={RelativeSource TemplatedParent}}" />

<TextBlock Text="{Binding Path=Info.MaxSpeed}" />

<TextBlock Text="{Binding Path=Info.MaxSpeed, RelativeSource={RelativeSource Mode=TemplatedParent}}" />

Car.Info = new CarInfo { MaxSpeed = 100.0 };