After what seemed like an eternity of stagnation - and sometimes even of regression - during the 1980s, 1990s, and early 2000s, average vehicle fuel economy of new vehicles sold in the U.S. (weighted by sales, so pretty representative) has finally sustained a healthy upward trend in the past few years. Someday it might not matter as much because most transportation will be electric, powered by clean energy, and more people will bike and take transit, but in the meantime, it's a very good thing for the cars that are on the road to be burning less fuel.
Here's what a couple decades of stagnation looks like. Note that the EPA has revised its testing methodology, so a car that got 25 MPG in 1985 wouldn't get close to 25 MPG on today's testing. This means that the numbers aren't directly comparable, but the trend certainly is (mostly flat for about 20 years, and now finally going up).
As you can see above, things are moving in the right direction. No single year had a huge jump, but if you add all those steps in the right direction together, you get a significant change. 5 MPG in about 6 years might not seem like much, but when you multiply those extra miles per gallon by the number of vehicles on the road, times the number of miles they drive each year, times the number of years that a car stays on the road, it's a pretty big number of gallons of gas saved. On top of that, Americans aren't driving as much as they used to...
For you number fiends, here's the same data as in the graph, but in a different format.
This one shows a few other variables and gives a better idea of the real impact of these changes. With the Eco-Driving Index (EDI) for March 2014 at 0.78, this means that during that month was a 22% reduction of emissions per driver of newly purchased vehicles compared to the situation in October 2007.
The blue line shows that people have been driving less compared to the reference year of 2007, and the red and green lines show the decline in fuel used and emissions.