Ah...so you carried an extra one hundred gallons(700 pounds of diesel) that you never used. If you would have carried 100 fewer gallons, your mpg would have been higher.
You can apply the exact same argument to everyone else who tops off the tank instead of carrying only the fuel needed for the trip.
By the way, airlines make a fine art of carrying only the fuel needed for their trip, plus legally mandated reserves.
zeusstx 
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