All the posts on fuel burn are useless unless you include the planned taxi fuel and planned flight plan weight. I have found over the years that if you subtract the unburned taxi fuel and were at the flight planned weight you burn right on the flight plan almost to the pound. If your lighter then the flight plan which is the norm then you under burn the flight plan. Fly fast and you over burn the flight plan however if you were under the flight planned weight it will mask flying fast and make you feel like there is no fuel burn increase. The 727 would go very fast but sucked fuel doing it. The 767 can also be pushed up to .83 and again burns a lot of extra fuel unless you are under the flight planned weight and under burn the taxi fuel. The flight planned weight is supposed to be the maximum you will see so generally your under that weight when the actual awabs arrives. Weight more then any other factor seems to affect fuel burn.