Key

$meta_query = array(
	array(
		'key'     => 'city_mpg',
		'value'   => '999',
		'compare' => '>=',
		'type'    => 'NUMERIC',
	),
	array(
		'key'     => 'mileage',
		'value'   => '2222',
		'compare' => '>=',
		'type'    => 'NUMERIC',
	),
);