One of the reasons at the heart of this gap is the ability for skilled test drivers to increase ratings by "smoothing out" the prescribed EPA test procedures. By carefully accelerating and braking, experienced drivers can save fuel, and gain an advantage for the test car in question. EPA spokesperson Christopher Grundler told AN that, "The industry has some very good drivers, and we've noticed."
As the average price of gasoline increases, and the importance to the public of accurate mpg numbers also rises, the EPA is seeing the necessity of making its test standards more stringent and more accurate. The organization is now requiring that automaker submit "drive trace" reports; electronic logs of driving behavior during testing. The EPA is also working with SAE International to further refine the process, and to help eliminate the affects of things like ultra-skilled test drivers.