This summer with our 2016 Volt driving down the Oregon coast on Highway 101 to go fishing we averaged 54 mpg just on gas and nearly 68 miles on electric for our 135 mile round trip. Now with temps in the upper 40's to mid 50's or so and rain, wet roads, mpg on gas the last trip, yesterday, to Portland Oregon via Highway 26 returned 51 miles on electric and 45.6 mpg just on gas, gas miles 117. Light to moderate rain and temps in the low to mid 50's and from sea level a total of 6 mountain passes, 3 each way, up to 1642 feet elevation. In comparison our 2010 Prius would have averaged, under the same conditions, 48-50 mpg.
Maybe we just lucked out with a more efficient engine than normal, if not it appears GM did their homework on improving the efficiency of the 2016-17 Volt's gas motor.