$sql = "SELECT (posledni.rychlost - predposledni.rychlost) FROM (\n"
    . " SELECT * FROM `data` ORDER BY `index` DESC LIMIT 1\n"
    . " ) AS posledni\n"
    . " JOIN (\n"
    . " SELECT * FROM `data` ORDER BY `index` DESC LIMIT 1,1\n"
    . " ) AS predposledni\n"
    . " ";