Study: Some car models consuming around 50% more fuel than official results
I monitor my mileage by simply resetting my trip meter on a fillup and looking at how much went into the car and dividing.  The largest source of error naturally is that not all pumps are gaurenteed to fill to the same hieght and some places may be on a hill etc.Obviously this is only as good as the way I drive (or my wife) and is heavily dependent upon if I hit traffic, mostly drive in the city or go on a road trip.It really is an interesting thing to keep track of.