Chevrolet Repair: Driver Information Computer, mesurment, system measures

Question
Question from fellow Allexpert (aviation):  The driver information computer in my 04 Impala consistently gives gas mileage readings that are way too high (avg. 35-36 versus actual 30-31).  The problem is that it under-measures the amount of fuel used.  I asked the dealer how or where the system measures fuel use (injectors, gas tank, fuel line?) but couldn't get a straight answer.  Where does that measurement come from?  Is there any hope of getting it adjusted, or is this feature just a toy, as the technician at the dealership told me?

Answer
as far as i know there is no fuel msurment in older cars but i wouldnt be suprised if there was some sort of doo hicky in a 2004 that would mesure that. the newer cars are getting to the point or being so computer run, it not funny. i was talking to a freind and he says there even designing tires that can only be changed by a dealership. that cuts ot us back yard guys to the quick. my guess is that that mesurment comes from the fuel tank or the sending unit but its probably not user ajustable. so it slightly short, at least you know the difference. odds are the techs dont have a clue and are just quoteing a manual or something till that feture is on most if not all cars and trucks. go figure, hopr i helped some