Car manufactures get their MPG for new cars going down hill with the wind. So, up hill toward the wind use more. Everyone gets different according to their driving habits. To know what you really would average. Fill up your tank and write down the Mileage. Drive until you almost empty and refill. Write down the Gallons it take to refill and the mileage. Subtract the first mileage from the second and divide that number by the number of gallons. This give you your MPG according to your driving habits.
Example;
Fill-up @ 10,000 miles.
Re-fill @ 10,500 miles
10,500 / 10,000 = 500 miles driven
20 gallons to re-fill
500 / 20 = 25 MPG
I hope my math is correct?