how I show selected value by metabox

I am wusing this code to add price and curency. But when I put $car_currency = get_post_meta( $post->ID, 'car_currency', true ); line in my page it show only car_USD not USD.enter image description here

class Car_Price_Meta_Box {

public function __construct() {

    if ( is_admin() ) {
        add_action( 'load-post.php',     array( $this, 'init_metabox' ) );
        add_action( 'load-post-new.php', array( $this, 'init_metabox' ) );
    }

}

public function init_metabox() {

    add_action( 'add_meta_boxes', array( $this, 'add_metabox'  )        );
    add_action( 'save_post',      array( $this, 'save_metabox' ), 10, 2 );

}

public function add_metabox() {

    add_meta_box(
        'car_price',
        __( 'Car Price', 'text_domain' ),
        array( $this, 'render_metabox' ),
        'car',
        'advanced',
        'default'
    );

}

public function render_metabox( $post ) {

    // Add nonce for security and authentication.
    wp_nonce_field( 'car_nonce_action', 'car_nonce' );

    // Retrieve an existing value from the database.
    $car_price = get_post_meta( $post->ID, 'car_price', true );
    $car_currency = get_post_meta( $post->ID, 'car_currency', true );

    // Set default values.
    if( empty( $car_price ) ) $car_price = '';
    if( empty( $car_currency ) ) $car_currency = '';

    // Form fields.
    echo '<table class="form-table">';

    echo '  <tr>';
    echo '      <th><label for="car_price" class="car_price_label">' . __( 'Price', 'text_domain' ) . '</label></th>';
    echo '      <td>';
    echo '          <input type="number" id="car_price" name="car_price" class="car_price_field" placeholder="' . esc_attr__( '', 'text_domain' ) . '" value="' . esc_attr__( $car_price ) . '">';
    echo '          <p class="description">' . __( 'Price', 'text_domain' ) . '</p>';
    echo '      </td>';
    echo '  </tr>';

    echo '  <tr>';
    echo '      <th><label for="car_currency" class="car_currency_label">' . __( 'Currency', 'text_domain' ) . '</label></th>';
    echo '      <td>';
    echo '          <select id="car_currency" name="car_currency" class="car_currency_field">';
    echo '          <option value="car_USD" ' . selected( $car_currency, 'car_USD', false ) . '> ' . __( 'USD', 'text_domain' );
    echo '          <option value="car_EUR" ' . selected( $car_currency, 'car_EUR', false ) . '> ' . __( 'Euro', 'text_domain' );
    echo '          <option value="car_GBP" ' . selected( $car_currency, 'car_GBP', false ) . '> ' . __( 'GB Pound', 'text_domain' );
    echo '          <option value="car_JPY" ' . selected( $car_currency, 'car_JPY', false ) . '> ' . __( 'Japanese Yen', 'text_domain' );
    echo '          <option value="car_CNY" ' . selected( $car_currency, 'car_CNY', false ) . '> ' . __( 'Chinese Yuan', 'text_domain' );
    echo '          </select>';
    echo '      </td>';
    echo '  </tr>';

    echo '</table>';

}

public function save_metabox( $post_id, $post ) {

    // Add nonce for security and authentication.
    $nonce_name   = $_POST['car_nonce'];
    $nonce_action = 'car_nonce_action';

    // Check if a nonce is set.
    if ( ! isset( $nonce_name ) )
        return;

    // Check if a nonce is valid.
    if ( ! wp_verify_nonce( $nonce_name, $nonce_action ) )
        return;

    // Check if the user has permissions to save data.
    if ( ! current_user_can( 'edit_post', $post_id ) )
        return;

    // Check if it's not an autosave.
    if ( wp_is_post_autosave( $post_id ) )
        return;

    // Check if it's not a revision.
    if ( wp_is_post_revision( $post_id ) )
        return;

    // Sanitize user input.
    $car_new_price = isset( $_POST[ 'car_price' ] ) ? sanitize_text_field( $_POST[ 'car_price' ] ) : '';
    $car_new_currency = isset( $_POST[ 'car_currency' ] ) ? $_POST[ 'car_currency' ] : '';

    // Update the meta field in the database.
    update_post_meta( $post_id, 'car_price', $car_new_price );
    update_post_meta( $post_id, 'car_currency', $car_new_currency );

}

}

new Car_Price_Meta_Box;

via Chebli Mohamed