The Volt seems to love going to Vermont, last week we went to Woodtock via Cornish NH, about 300 miles round trip, the Volt got 48MPG. Thought that was pretty good, but this week went to Brattleboro, only about 175 miles roundtrip, got 53.6MPG. I always drive in L, use Hold on the way out after consuming about two bars, Mountain on the way back. Regen braking picked up a full bar on the way out which certainly contributed to the great number.
Your results aren't really surprising.
300 miles at 40mpg is 7.5gallons.
300 miles at 48mpg is 6.25 gallons.
1.25 gallons at 40mpg gives 50 miles.
175 miles at 40mpg is 4.375 gallons.
175 miles at 53.6mpg is 3.265 gallons.
1.110 gallons at 40mpg gives 44.4 miles.
Thursday we escaped the humid 90s by heading far Downeast, to Winter Harbor (74*), Schoodic Point (low 70s by the water) and Lubec (69* by the water).
All away from the Interstate and lots of 55mph driving. Our 2013 was getting a little over 40mpg in hybrid mode, but stealing a bit from EV since we got a little over 38 miles EV range.
But, since I know we'd have beaten our mpg with our 2010 Prius it doesn't really impress me. What matters is that this is _exceptional_ driving, day to day driving is mostly electric, and that this was just with a Gen 1. Plug-ins are improving measurably with every generation.