The trend just looks erratic because it is scaled between the minimum and maximum values of the last ten tanks, rather than between zero and the maximum value. So no matter what your variation between tanks is, Fuelly will scale the graph to make it look like values are changing wildly.
Click on 'chart all fuel ups' to see a less erratic graph, in which you usually see seasonal changes from that I'd say that you live on the Southern hemisphere because the winter (I mean our winter, around January) has better average. But you don't, do you?
Yes, thanks.. that's the one I usually use, or the histogram for a month to month average.
I live in the south of North America, Florida. I've noticed others getting worse mileage averages in their Corollas, and they're generally located farther north, where it is bitterly cold. Mine gets worse in the summer, and their's gets better. Air conditioning kills my mileage in the summer. Snow and slush and prolonged idle time wrecks their's.
It's easy to see in the histogram where you compare monthly averages.
AC? That makes sense. I don't have AC in any of my vehicles, so it won't affect me (but riding Teresa I can have a lot of wind, no matter the season ). Snow and slush... yes, I get these, pretty often lately. This morning too.