I'm too lazy to keep up with the daily EV fuel log for the Volt, but when I had my Insights I would top off the tank at the same pump at the same gas station at the same time of morning, recording the miles driven since last fill up and the amount of fuel added at the fill up.
Since the Insight would go about 700 miles on a tank, that's about as accurate of a number as you can get in a car. When you drive 712.6 miles and divide that by 9.52 gallons, it's pretty accurate and simple math to get an overall mpg average
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