How to get the db table row that has the biggest value in the meta value?

The WP_Query has a feature called meta_query. Using it allows you to either query by value or numeric value:

$car = new WP_Query( array(
    'post_type'      => array( 'cars' ),
    'meta_key'       => 'car_number',
    'orderby'        => 'meta_value_num',
    'posts_per_page' => 1,
    'order'          => 'DESC',
    'cache_results'  => true,
) );
$carNr = get_post_meta( $car->ID, 'car_number', true );
// Inspect result
var_dump( $carNr );

Now, that we got the right post type, the appropriate meta key and defined that we want to order by its value, we only need to limit the result to a single post and have a descending order. As we now got the post, we can easily grab the meta value.

Another option would be to do a plain query:

global $wpdb;

$wpdb->get_var( 
    "SELECT {$wpdb->postmeta}.meta_value
     FROM {$wpdb->postmeta}
     WHERE 1=1
         AND {$wpdb->postmeta}.meta_key = 'car_number'
     ORDER BY ABS( {$wpdb->postmeta}.meta_value ) DESC
     LIMIT 0,1"
);

Depending on how you saved your values – often plugins save numbers/digits/integers as strings – you may need to convert your value during the query. In that case replace the order by part with

CAST( {$wpdb->postmeta}.meta_value as SIGNED INTEGER )

Note: If you don’t have negative integers, you can go with UNSIGNED

CONVERT( {$wpdb->postmeta}.meta_value, SIGNED INTEGER )

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