Before I answer that question I have to tell you a few things. I bought the car in order to support my website http://MyPerfectAutomobile.com . So I'm driving it like I would a regular car, but also test it to see how it works under different circumstances. So in order to see how it reacts when out of battery I deliberately did not charge it on the second day I owned it for a few days and drove hundreds of miles. Naturally my Volt had to use gasoline those trips. I'm also not a Hypermiler (trying to squeeze the most out of a gallon or the most out of a charge).
Right now I'm getting about 28 miles to 30 miles on a charge. This is all highway driving at 75Mph. When I'm driving in the city it has a chance to use the regeneration and the mileage goes up. I know this because when the Volt is fully charged it will tell me the range based on my driving habit. Last week instead of flying from San Jose, CA to Portland, Oregon I drove my Volt to see how it would drive, a 10 hour trip non-stop. I had the cruise control at 78 Mph most of the way in CA. In Oregon they seem to drive slower so I had to lower the cruise speed. The Volt drives like a luxury Cruisemobile. Very comfortable. Like a Buick Electra. ( I like that name, appropriate). I tested to see how fast it would go but the limiter came on at 100Mph with a message on the console saying that the Volt is limited to 101Mph.
In Mountain mode it will charge the battery deliberately using the engine and even when the battery was empty at the end of your trip there will be 13 miles added when you turn Mountain Mode off.
One of these day I will drive it just in the city and see how much I get in the city. I know from experience it will be a lot more. The regeneration will add tons of miles for sure.
Bob T
http://MyPerfectAutombile.com