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2020 2500 AT4 Diesel Fuel Economy after fill up

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I have a 2020 GMC 2500 AT4 Duramax Diesel.  Fuel economy is normally 18-20 mpg local and 21-25 mpg highway.  I've noticed multiple times that if I completely fill the tank up, that my instantaneous fuel economy and average drop significantly.  Instantaneously runs about 8-12 mpg and my average over the last 50 miles drops down to 11-13 mpg.  Then suddenly after about 25-50 miles of driving, it will jump right back up to good fuel mileage.  If I never completely fill the tank, this doesn't happen.  Dealer says they've never heard of this, but I don't trust dealership feedback.  Previously I had reported multiple times that DEF fluid levels were completely wrong and erratic and they said there were no reports about it.  Then miraculously there was a software recall that fixed it.  

Edited by Joeroberts362
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