Though vehicles for sale today are more efficient than they've ever been, the average fuel economy of America's operating fleet has remained relatively stagnant since George W. Bush was President, research shows.

Researcher Michael Sivak, writing at Green Car Congress, puts the focus on the miles driven by registered vehicles in the United States and points out that periods of great growth for new-vehicle fuel efficiency haven't made a demonstrable impact on the immediate measured average fuel economy of vehicles currently registered and operating.

Instead, they lag by several years. As Sivak points out, even in years of record sales, only a small percentage of the fleet turns over. In 2017, when more than 17 million new cars and light trucks were sold in the United States, those vehicles accounted for less than 7 percent of the total vehicles registered in the United States.

The shift toward larger trucks and crossovers in the years since 2008 hasn't helped either. Despite the fact that these larger vehicles are vastly more efficient than their predecessors, the sheer volume in which they sell is going to influence total fuel consumption figures.

It has been eight years since the Obama administration announced its CAFE road map, and with just six more to go until CAFE mandates 54.5 mpg for the U.S. fleet average (which is measured differently than actual consumption, or even EPA estimates), the current administration has been fighting to roll those standards back.

The 2020 election will likely have a direct impact on the future course of fuel economy standards, especially since it will likely act as a referendum on Trump's environmental policies and determine whether legal action continues between the federal government and the state of California in the ongoing battle over the state's authority to set its own fuel efficiency standards.

In other words, these vehicles, and their emissions will last a lot longer than four more years.