Can't override magento core config model

Can't override magento core config model Mage_Core_Model_Config. I have magento 1.9.2.1. Here is config.xml

<global>
    <helpers>
        <peexl_customflatrate>
            <class>Peexl_CustomFlatrate_Helper</class>
        </peexl_customflatrate>
            </helpers>
    <models>
        <peexl_customflatrate>
            <class>Peexl_CustomFlatrate_Model</class>
        </peexl_customflatrate> 
                    <core>
                        <rewrite>
                             <config>Peexl_CustomFlatrate_Core_Config</config>
                        </rewrite> 
                    </core>    

And class Peexl/CustomFlatrate/Model/Core/Config.php

class Peexl_CustomFlatrate_Model_Core_Config extends Mage_Core_Model_Config
{

}

Nothing happens :(