Your calculations are incorrect, you need to use total miles.... Do not subtract anything, just total miles divided gallons filled ... 131.5/ 5.059 = 25.9932793042... that is the "real" math and real MPG calculations, you beat the rating and easily beat the 3.5 eco boost. They get around 20...
First to know is mpg is for total miles: 131 / 5 = 26 mpg (or close to the calculated). That meets or beats the EPA stated MPGs by approximately 2 MPGs.
The "gas" miles is not the correct way to think about it, the added weight of the battery and other equipment would give a lower "gas" milage...