The way I calculate mine is like this:
I purchased the car with a full tank. I write down the mileage and date. I run it out of gas. I fill up and write down the gallons I put in, the new mileage and the date. I run it out of gas and fill it up. Write down the gallons, the new mileage and the date. I repeat and repeat. I post all the information in
www.fueleconomy.gov and here is what I get. Do you have anything similar?
Note: there are three gaps where I didn't follow procedure. You can tell where the line is longer than normal.