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).
GM has also found that of all the overall driving done by Volt owners, 63% of the miles are driven in electric mode, and an independent study found that for 81.4% of trips the gasoline engine didn't even turn on.
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.