Mileage declining ... must be winter?

Here is chart of my fuel economy which included summer temperature and start of winter temperatures which came with lots of stop and go traffic. Between the stop and go and the car trying to stay warm. Still way better than my old car which I sent to the scrap yard in July. Since July 24, 2012 I have spend $881.42 less on fuel I would have on my old car for traveling 11930 km (7412.96 miles) within 4 months.


The upper and lower control lines was calculated using +/- 3 x Standard Deviation from the average actual economy.

 

Nice work on the chart.
Your numbers are pretty well exactly the same as mine since I took delivery of my v august 28th. I also have around 12000km on my car. The cold weather warm up and heater use does have a big impact on fuel economy but as you say its still better than most anything out there.
Even since the cold weather hit my worst tank has still been as good as the best I've ever gotten in my old car(a subcompact).