Based on this article’s math, if a person drives only 30 miles per day, then this car will never run out of gas, and will get an infinite mpg.
Assuming you plug it in every night, it would never use any gasoline.
Of course, that’s not free but as far as gasoline goes, it is infinite mpg, since you’d never use any.
On the other hand, if you never plugged it in, after the battery went flat, it would use gasoline all of the time and you’d have a 60mpg car lugging around a heavy dead battery pack.
Basically, if you buy a Volt, you will need to plug it in every night for it to make any sense to own it.