If you rarely drive more than 50 miles before charging the battery, you can get away with driving your Volt on as little as a couple of gallons per year. Hence, the 6,200 MPG is achieved.
Most people, however, have to drive a little longer than 38 or 50 miles from time to time. At that point, your driving blend will take your MPG down to a level that depends on your mix of driving and ability to charge the battery.
Chevrolet publishes the "all-electric miles" as part of the 100-million-miles driven cumulative to date, and that number is 63%. What does that mean for the truly fleet-wide MPG? If 37% of your driving happens at 38 MPG and 63% at infinite MPG (electric), then one gallon takes you (100/37 x 38 MPG) = 103 miles. So, 103 MPG would be the cumulative fleet-wide mix.
This obviously doesn't account for the electricity consumed. While that is something I and many other people have written about many times before, suffice it to say that basically you drive around 3 miles per kWh. Since the nationwide price of electricity is around 11c per kWh, it would cost you $440 to drive 12,000 miles purely on electricity (zero on gasoline).
The Volt's Reliability, by the NumbersWith 100 million miles driven in the Chevrolet Volt, what has been the reliability? By all accounts, it has been fantastic. I am unable to find more than the most random and rare issue having been reported. If you can find some examples -- any, even the tiniest glitch -- let me know. I really would like to know.
Why has the Volt had such superior reliability? The reason is similar to why a computer with a solid-state drive has a better reliability than one with a hard disk drive. There are simply fewer moving parts. The electric motor has almost none of these items: belts, hoses, oils, spark plugs, fans, valves, distributor, transmission, and so forth. But the Volt also has a gasoline generator, you say? Yes, and as a result, it has all or most of those moving parts listed above. The counter argument is that a Volt-style gasoline engine, when it operates as a generator, as opposed to being tied to the drive shaft as the main means of propulsion, can be run in a more gentle and controlled manner, putting less strain on the system end-to-end. You can see this in the lack of a traditional transmission in the Volt, for example -- it's much simplified!