12-month Average

Because I felt that my Lifetime Average was starting to flatten out, I decided to add a 12-month average to my spreadsheet and chart. In the last year, it's causing me great confusion. Perhaps I just need a reality check.



As you can see, it's dropping like a stone. I can't figure out if that's what it's supposed to do or if I'm calculating something incorrectly. Granted, I don't have perfect year-to-year dates so I try to get the dates ranging from "closest to one year ago until today."

So my question is this: am I doing this correctly and this dramatic drop is correct or am I doing something incorrectly and I need to fix my math?

For all the numbers, hit the link in my signature.

 

Here's the entire spreadsheet.

 

No guys, but thanks.
I know exactly what's going on with my car and I am comfortable with the regular maintenance that I maintain. I was exclusively referring to the math of the spreadsheet.

 

It's just that, well, you can see how it's dropping so dramatically in the last year. I figure that it'll probably swing back up but I still find it disturbing.

The reason the winter mileage was so low last winter is that I took an "I don't care" attitude." That lasted until I chalked up my first really bad tank and realized that I do care after all.