My 1500 weights more then 6,000 even without me sitting in it.
33 gallons of gas weighs around 200 give or take a little depending on the temperature...
If I fill up only to a half a tank I can go from 6360 to 6260. It would make no difference
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You ordered an fully loaded Rebel. You can forget getting any kind of good mpg, that will get even worse then my Limited. Wrong choice of truck if you were concerned about gas. Maybe cancel and re-order a Bighorn or Tradesman with no features (as those add a bunch of weird way more then a fuel tank).