The fuel app is great. I use https://www.fuelly.com/
. Smartphone, iPad or computer is easy.
As for the question of fuel. If your within generally 400 miles of a refinery, they usually deliver from the source. But generally further, when and order is placed, it goes to the local spot market station and they blend and ship to the specs needed. This includes seasonal additives as needed. You have no control over this. (summer blends and winter blends are different and does effect mpg's.) Also, if the blend is off, your stuck with this.
As to weather and road conditions, you basically have no control over. You play the hand you are dealt, and adjust as needed.
The overhead lie-o-meter is usually off or it wouldn't have achieved this nick name. But if yours is like mine, it's a constant amount off and I only use it as a guide to the increase or decrease in tenths. If dropping and not going up a grade, I need to adjust. If dropping slightly it can also inform me that there is a slight grade and adjust as needed. Means a lot on a vacation 5k round trip in gallons.
There are two areas that you have some control over.
* Maintenance of your vehicle:
If you maintain it properly, you might not like the mpg's, but they will be better than if you neglect it. Minor changes mean a lot.
* Foot to the pedal;
It was mentioned above and this is usually the main demon. Most never think of this, but your emotions at the time your foot hits the pedal determines what mpg you will get. If you pi**ed, happy or such, the pressure is different than if you want great mpg's.