So, to get an average of the data from my excel-sheet, I can only have two fuel-ups? And what do i do about the volume of fuel, what do i put in? Or can i have 40-ish fuel-ups, with odometer readings at both ends, and then just have blank odometer-readings?

That's just it—the data you have is not going to work here at Fuelly. I'm sorry about that, but I don't think there's any way around it.

You might want to summarize your current fuel economy findings from your spreadsheet and enter it as a note here at Fuelly. That will tell others what you've learned from your personal tracking. Other than that, I don't think there's a good way to use your data here at Fuelly.

Where should i enter a note, just as a note linked to a specific fuel-up?

On the left side of the page under "Add a fuel-up" is an option to "Add a note". Click that to add a note to your vehicle.

Thanks for the help! :)