Thank you for your in depth explanations. Let me give you a better example of what I'm not quite sure of. Our airplane holds 15500# of fuel. We landed after a 4.5 hr flight and asked for the airplane to be filled up. When I got back into the plane to start it up and program the FMSs the fuel gauges (one for each of the three tanks) added up to 15800#. My question is, should I enter 15800, or 15500. How much gas do we really have, and if the gauges say 15800, will we be able to go further on that, or do we still have the same amount by volume.

Thanks for all the info. You guys are too smart for me so you may have to dumb it down a bit if you can. Thanks again.