Odoo - one2many sum

I'm working on simple project and I've got a problem. I want sum one column in my one2many fields how i can do this ?

from openerp import models, fields, api, _


class Fam(models.Model):
    _name = 'fam'

    fm_id = fields.Many2one('fam')
    mileage = fields.Float(string="Mileage", required=True)
    fueled = fields.Float(string="Fueled", required=True)
    perliter = fields.Float(string='Price per liter', required=True)


class Car2(models.Model):
    _name = 'car2'
    _description = 'Car record'
    _log_access = True

    name = fields.Char(
        string='Name',
        required=True
    )
    mile = fields.One2many(
        "fam",
        "fm_id",
        string='Mileage, Fuel and cost perliter',
        required=True
    )
    average = fields.Float(
        string='Average'
    )
    combustion = fields.Float(
        string='Combustion'
    )