Weight will be pretty important in regards to fuel economy in addition to a bunch of other factors like driving habits. Personally the 64 km all-electric range of this car is more than enough for myself thus it would seem wasteful to drive around with a full gas tank.
Any word on how big the volt's fuel tank will be relative to current models?
Do you guys think it would be prudent to drive low on gas, or may I even dare to say: on an empty gas tank? And would the increase in mileage be worth it?

Weight will be pretty important in regards to fuel economy in addition to a bunch of other factors like driving habits. Personally the 64 km all-electric range of this car is more than enough for myself thus it would seem wasteful to drive around with a full gas tank.
Any word on how big the volt's fuel tank will be relative to current models?
Do you guys think it would be prudent to drive low on gas, or may I even dare to say: on an empty gas tank? And would the increase in mileage be worth it?

I heard that they are dropping the planned range down to 300 miles, leaving the range-extender to provide 260 miles. It's hard to say how big the tank will be, because they claim 150 mpg's sometimes, but I am guessing it would be anywhere from a 3 gallon to 6 gallon tank. I agree with you that I would probably only keep it under half full, unless I was planning a long trip.

GM has placed 4 factors in order in regards to their effect on the Volt's range,

1) Aerodynamics
2) Power consumption by accessories (AC, radio, defroster, etc.)
3) Weight
4) Tire rolling resistance

I estimate an 8 gallon tank for the Volt. The weight of 8 gallons of gas is about 50 lbs. For the difference between 3000 lbs (no gas) and 3050 lbs (full tank), I bet you would be hard pressed, even in laboratory conditions, to tell the economy difference based on the added weight of the fuel.

Conclusions: Drive it with whatever amount of gas you want, the effect on negligible.

Several of the recent articles after last week's press junket mention that they have decided to reduce the tank as Jason mentions, but I recall reading they expect 400 miles range. They have been stating about 50mpg fuel economy in generator mode, which would mean about 7 gallons. They mentioned saving space, weight, and money but it may also have to do with stale gas. They have repeatedly mentioned this as a concern and the bigger the tank, the longer a full tank will last.

I haven't heard of nor can imagine issues with partially filling the tank but the engineers did seem to shutter at the thought of no gas. That could cause issues.

GM has placed 4 factors in order in regards to their effect on the Volt's range,

1) Aerodynamics
2) Power consumption by accessories (AC, radio, defroster, etc.)
3) Weight
4) Tire rolling resistance

I estimate an 8 gallon tank for the Volt. The weight of 8 gallons of gas is about 50 lbs. For the difference between 3000 lbs (no gas) and 3050 lbs (full tank), I bet you would be hard pressed, even in laboratory conditions, to tell the economy difference based on the added weight of the fuel.

Conclusions: Drive it with whatever amount of gas you want, the effect on negligible.

I'm a little concerned that the list does not include electric motor efficiency and regeneration recovery efficiency.

I got the energy priorities from the following link:

http://www.autobloggreen.com/2007/12/12/volt-aero-and-styling-touring-the-e-flex-design-studio-and-gm-w/

Here is the quote of interest:

"Driver behavior is clearly a factor for both traditional and electric cars. Beyond that, on traditional cars, factors like mass, aero drag and rolling resistance come into play in that order. In testing and simulation GM has found that for electrically-driven vehicles mass actually drops to third on the list behind aerodynamics and electrical loads with rolling resistance coming in fourth."

I'm for making the tank smaller. Less space, weight, cost and a better turnover of little-used gas. Besides, who needs the 600 mile range? People say that today's cars have 600 mile + range. That may(is) be true but they use a lot more gas during the week than the Volt will. I think they should size the tank for the average highway driving time between bathroom breaks. :) I don't know about you marathon drivers but I'm good for about 4 hours at a time max. After driving 4 hours I like to stretch my legs. So, in my example in 4 hours at 70 miles per hour I get a tank size of 280 miles of needed range! Heck, that 400 mile range is looking good to me. :) I know several factors determine the range but I think for a plug-in hybrid vehicle the tank sizes can be significantly smaller.

Several of the recent articles after last week's press junket mention that they have decided to reduce the tank as Jason mentions, but I recall reading they expect 400 miles range. They have been stating about 50mpg fuel economy in generator mode, which would mean about 7 gallons. They mentioned saving space, weight, and money but it may also have to do with stale gas. They have repeatedly mentioned this as a concern and the bigger the tank, the longer a full tank will last.

I haven't heard of nor can imagine issues with partially filling the tank but the engineers did seem to shutter at the thought of no gas. That could cause issues.

Oh yeah, stale gas would be a good reason, along with saving weight and energy lost through sloshing fuel (I know, its negligible). Looking at the latest wind tunnel design, reduced fuel tank, etc., this Volt is going to be a great vehicle. I am not sure why a 2 + 2 isn't a popular configuration, but I think a lot of people will still like it.