About a year and a half after reaching the milestone of 100 million electric miles, the Chevy Volt owners have passed 500 million electric miles, no doubt on their way to a billion sometimes in 2015 or early 2016 at the latest. That's a lot of gasoline, equivalent to more than 25 million gallons, or to put it another way, about two-and-a-half months of gasoline use in a city the size of Washington DC!
Having a large fleet of plug-in vehicles out in the real-world gives all kind of interesting data on how these really perform, which is often different from what the EPA tests would predict. One such data point: GM has been tracking 300 Volts in California for the past 30 months, and about 15% of them get more than 40 miles of electric driving per charge (EPA rates the Volt at 35 miles of EV range).
This truly confirms not only that a plug-in hybrid can make a big difference despite a relatively small electric range, but also that most trips are short and most people don't need huge driving ranges as long as there are public fast-charging stations available for when longer trips are needed (in other words, a lot of people would be just fine with the limited range of the Nissan LEAF, and will be more than fine when most mass-market electric cars reach 150-200 miles of driving range in a few years).
The extreme example of this is the Volt owner who drove 12,000 miles on 26 gallons of gasoline.