Fuel economy for vehicles in the U.S. spent about 20 years stagnating, not because human ingenuity was exhausted and we couldn't find a way to squeeze more out of each drop of fuel, but because incentives to improve were either absent or small (low oil prices, little or no pressure from regulators, etc). As you can see in the chart below, after a big spike in the late 1970s-early 1980s, things plateaued, and even went backward, between about 1985 and 2005:
Then high oil prices (hurricane Katrina woke a lot of people up!) and new CAFE fuel economy regulations got things moving again. The chart above shows this up to 2010, and the one at the very top of this article zooms in on the years 2008-2015. As you can see, things kept improving fairly rapidly until 2014.Going from 20 MPG to 25 MPG is a 20% improvement, and over a vehicle fleet of many millions of vehicles, that's quite a lot of fuel that isn't being burned! It's not enough, and ultimately we need to burn zero fossil fuels, but in the meantime, we certainly shouldn't waste any as the EV and clean power grid transition is under-way (and even better if you can walk, bike, or take transit!).
2015, so far, seems to show a new plateau. We've been going backwards a little compared to 2014, probably because of the oil price crash (I wrote about the pros and cons of having lower oil prices here). Hopefully that's just a temporary bump in the road and not new extended period of stagnation.
Above is another chart from the University of Michigan's excellent Transportation Research Institute, showing the trend in three different data sets.
The xmas lines (red and green) show how much fuel is being used per mile driven and how much CO2 is being emitted per mile driven. They obviously trend together, and the direction is what we want to see (down!). But the blue line, which shows distance driven is starting to go up after plateauing for the past few years. This is more worrisome, as ideally we would see it go down as more people find alternatives to driving for most of their trips.