How can I display the average of the speed field, instead of the sum?

XML TREE

        <field name="arch" type="xml">
            <tree string="Models">
                <field name="deviceid" />
                <field name="devicetime"/>
                <field name="speed"/>
                <field name="address" />
                <field name="event" />
                <field name="leido" />
            </tree>
        </field>

XML SEARCH

        <field name="arch" type="xml">
            <search string="Vehicles costs" >
                <group expand="1" string="Group By">
                    <filter name="groupby_devieid" separator="1" context="{'group_by' : 'deviceid'}" string="Vehicle"/>
                </group>
            </search>
        </field>