A great chart and I would be the first to "champion" it, but really, is the difference even statistically significant?
It was not until October, 2014, when Saudi Arabia said it was going to "open the spigots." The MPG had actually dropped prior to that. One can argue the fine points.
But the best MPG appeared to be around 25.8 mpg. Now, more than a year later with significantly lower gasoline prices, MPG for US vehicles averages ... drum roll ... 25.2 mpg.
For the full-year periods (October, 2013, to September, 2014; October, 2014, to October,2015), the MPG remained exactly stable: 25.3 mpg. October, 2013: high gasoline prices. October, 2015: low gasoline prices).
I'm not sure I would call a drop from 25.8 to 25.2 "deteriorating.
If one goes back to 2010 when the Bakken was hitting its stride, the MPG has increased from about 22.0 mph to 25.2 MPG. That seems significant.