Newer BMW's have for sets of data:

Since New
Since Refueling
Trip (resets after the car's been off eight hours).
Individual (reset by user)

Older BMW's had two: Trip, and On Board.

I record and reset the On Board values when I refuel. I also record my fuel purchases. From that data, I can calculate the errors of the displayed MPG.

You can't trust gas gauge accuracy. I've adjusted the displayed MPG's Correction Factor. Accuracy for both of my BMW's are now within 0.5%.

Using the trip odometer (reset at refueling), On Board MPG, and the Correction Factor, I can run my tanks down to less than one gallon without worrying about running out of gas. My estimates of how much fuel I'll need is usually within 0.1 gallons of what I actually need.

I have to make an effort to burn a tank of gas out of my 535i is less than 30 days. So, I like to run the tank to almost empty to get rid of most of the old gas.