My main concern is how the car actually drives when the battery runs out, since I will still drive it a lot on gas only mode.
The Volt has a 150 hp electric motor. Top speed of 100 MPH is all-electric. This means the Volt really drives like an electric car, even when it switches over to gas mode. The main thing you'll notice is the disconnect between the accelerator and the gas engine. For example: If you're stopped and you floor it, the Volt accelerates instantly, jumping off the line, then after a second or two the gas engine kicks in, but by then you're already going around 25MPH.
Does the engine turn off when coasting, stopped, low speed driving?
Coasting: Yes, the engine shuts off at lower speeds, maybe less than 35 MPH, I'm not sure.
Stopped: Yes, the engine is generally off.
Low Speed Driving: Yes, the engine usually cuts off below 20 MPH or so.
I'm just worried that I'm so used to my Prius it will feel odd in city driving when my battery is depleted. My V wagon will shut down the engine anytime you take your foot of the gas below 46 MPH and can coast/drive along silently.
It will feel different, but in a good way. It really feels more like an EV than a hybrid, even in gas mode. Electric grin...
How many gallons do most of you actually put in the tank when you fill up from empty?and would running mid grade be okay?
Gen1 Volts specify premium gas. I always use premium, and I always fill it up. For the amount of gas we use, the price difference is minimal. I've also heard premium gas is less corrosive. Keep in mind that many Volt owners go several months between fill-ups. In fact, some Gen1 Volt owners have exceeded 20,000 miles on a single gallon of gas. So stale gas can become an issue. The software in the car will automatically run the engine every few months to keep it healthy, and use up stale gas within a year or so, but cheap regular gas may be going stale by then.
Similarly, Volt oil changes may only be every 2 years, depending on your driving pattern, according the oil-life software built into the car. But GM specifies pure synthetic oil, and I always use pure synthetic oil for longevity.
In other words: Since the gas engine may go long periods of time without use, higher quality gas and oil make a difference.
would it achieve 40 MPG on a 200 mile highway trip at 70 mph?
Let's see, if you start with a full charge, you'll drive the first 38 miles gas-free. For the reamining 168 miles, you'll get around 37 MPG, which means you'll use 4.4 gallons for the 200 mile trip, which averages out to 46 MPG.
I drive from Nashville to Alpharetta GA pretty often and my car currently gets 43 mpg doing that.
I often drive from Northern NJ to Washington DC area. The Volt works extremely well for long trips. The extra weight of the battery makes is ride like a much larger car. Very smooth. Seats are firm, no back fatigue. Interior is very quiet. Note: For longer trips, I often use "Hold Mode". So I start out with 38 miles of electric range, but I save that for traffic jams, residential roads, traffic lights, etc. In particular, stop-and-go traffic is a lot less annoying in pure electric mode, without the engine starting and stopping constantly. Then before I reach my destination, I switch back to normal mode to use up the EV range for max MPG.
I've been lurking for a long time now. Finally a member. Thanks in advance!!

-Aaron
Hope this helps,
Dave