On my 2002 Xsara hdi 110, the computer screen in the middle states mpg, trip etc etc, and i have never thought much of it. But i discoverd by flicking through menus on it, there is a menu labelled Overdrive, i have 3 options, On, Off or program, but what is it? I dont have a handbook im afraid to say and havnt the slightest clue what it is, i know its not going to be a gearbox overdrive like cars from the 70's Any help is appreciated. Thanks.