I recently acquired a 2013 myself and it seems in-line with what I am seeing. I'm doing much of the same stuff, 40 psi, etc, but certainly don't have to deal with 5°.... have it ever reached that in middle of Alabama?
First, I don't count the charge sustaining MPG when the engine just kicks in for a few minutes. Thursday I was running a bunch of errands getting ready for oral surgery the next day and ran out of battery 1.6 miles from home. I bet that "trip" returned well less than 20 mpg because the first minute or so was, the ICE was using more fuel until it gets warmed up.
I take a frequent East-West road trip across Alabama. In nice temperatures, 60's and 70's, minimal climate control, I'm only seeing 27 or 28 mpg with a mix of speeds ranging from a 50 mile leg at 70 mph to a bunch at 55 mph.
The last trip, I avoided the Interstate leg and took the two-lane. It was a nice day in the upper 60s and enjoyed the leisurely drive. I pulled out of my driveway and drove five miles on battery. When I reached 55 mph, switched to Hold mode, waited about a mile and then reset Trip B. 165-ish miles later, nearly 100% at 55 mpg and having only driven through 4 small towns (only two big enough for traffic lights,) I arrived at my destination town when I switched back to battery for the remainder of the drive. Trip B was reporting 28.6 MPG which was near a record for me.
However factoring in another 20 miles on battery that day and another errand, I easily exceeded 30 MPG overall for the "tanks."
(My 4WD Silverado on that exact same drive one week later with similar weather conditions returns 23.8 MPG resetting the drivers information center at the same spot and checking at the destination city limits. My 2013 Malibu was always around 41 mpg for the entire trip, regardless of the exact route.)
Anyhow, I, too, wished my mileage on gas was better on the Volt and that it didn't require premium unleaded, here often 60 to 80 cents per gallon more than regular, but I have to remember that I have driven over 600 miles without gas at all made a tank of gas last over 1000 miles. In the end, I am driving 600+ miles per month on electric and keeping even more of this miles off my truck which I love.... despite only getting 16-18 mpg around town.
That said, I agree. When your weather warms, the engine will use less warming up, warmer air is less dense and thus has lower resistance, you'll need less power for climate control (although once the ICE warms up, you'll have plenty of heat) and you'll lose the winter blend of fuel. It sounds like you'll do just fine in a few months.
Now I need to go outside and check on my tomatoes. We're suppose to have a cold snap here and get into the 30s... (grin)