The gap between the estimated fuel economy demonstrated on a car's window sticker, and that which is achieved while driving in the real world, has always existed. Further, there has always been a difference between the mile-per-gallon numbers submitted to the Environmental Protection Agency by automakers, and those that the agency has garnered for itself in independent testing. In fact, according to an Automotive News report, the EPA believes that gap has gotten "unacceptably high" over recent years. The difference between EPA and automaker estimates was 1.15 percent in 2007, and jumped to 2.25 percent in 2010.

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.