Popular Mechanics Magazine tes drove a Chevy Volt in the state of Michigan. Road conditions involve city and state highway, (where the average speed is 78 mph).
source..
900 Miles Reliablity Report in the Chevy Volt Larry Webster. Popular Mechanics. Oct 10, 2010
Determining the Range on the Chevy Volt Larry Webster. Popular Mechanics. Oct 10 2010
Charge Sustain Mode (gasoline engine turns on) City MPG = 31.67 mpg Highway MPG = 36.0 mpg
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