I read a lot of folks on here who claim they are easily getting 40-45 or even higher MPG when running on gas. I’ve had my 2018 about six months and driven about 7,500 miles with about 2,000 miles on gas - mostly over a few longer trips but a handful of short trips around town beyond the battery range for the day. I’ve found I’m usually getting only about 39-40 mpg. My Voltstats figure shows my lifetime MPGcs at 39.0, yet that still shows me in the 69th percentile meaning I’m still getting better gas milage than the large majority of Volts out there (granted, I think Voltstats combines Gen 1 and Gen 2). I’m really curious how people are getting 45 and higher MPG on gas. I do drive moderately aggressively, but it doesn’t seem to matter. Even when I drive slow and steady around town (hybrids do better around town than standard ICE), I still barely crack 41 on a good day.
It takes 5 to 8,000 miles for an ICE engine to break in. During this time the engine efficiency will improve. The time you're on battery doesn't count towards this break in. Also, boost your tire pressure to 40 PSI (you can go as high as the Sidewall pressure but the Volt's tire/suspension combo really gets harsh above 42 PSI or so on the OEM tires).
Sheldon's graph of MPH vs. MPG is basically right. My experience is my MPG tends to run a little higher, but I live in Colorado with thinner air and serious hills. The Volt's downhill regen is better than 80% efficient so I get a lot of natural Pulse & Glide hypermiling improvements even when running at a steady speed on cruise control.
On my two round trips from Denver to New Hampshire I averaged 38-40 MPG while running on gas. On long road trips I set the cruise control at the highway posted speed limit or one - two over as I've discovered that's as fast as it's safe to drive across many States. In cities and towns I set it to the posted speed limit.
I agree with UtahK, if your local "regular" is 85 octane, either find a pump with 87 octane or mix your own while filling. GM's Ecotech line of engines do NOT like 85 octane.