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That's a very interesting trendline!
Off of the top of my head:
-I've always seen my MPG going up during my ownership of a car, but the longest period I followed it for was 18 months / 20 000 miles. That might be down to learning the car, but it would plateau after two years at the most, not keep going up like yours
-Global warming maybe? A few milder winters & warmer summers may help the engine run cold for shorter amounts of time?
-Oil? Have you always used the same?
-Has there been a no-speeding policy enacted over the last decade that may have pushed you to driver closer to the limit?
-Maybe fuel providers have increased the quality of their product so that it plays nice with DI?
-Driver weight? Did you gain / lose weight?
It might also be down to completely external factors, like you gas station having its flowmeters fine-tuned to read closer to the amount of fuel delivered.
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