Fuel Economy differences on 1st half of tank

Joined
May 23, 2000
Location
Bloomington, IL, USA
I have experienced a significant decrease in overall fuel economy if I wait to fill near empty. MPG ~ 45-47
...

However, if I fill at half of a tank remaining, my fuel economy numbers are more like 53-55 mpg


I vent each time and top off to make sure my calculations are accurate!

What would cause a shift in mpg in the latter half of the tank?
 
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