The 2017 Volt on gasoline only is rated approximately 5.6 L/100km (42 mpg). The 2017 Prius Eco is rated 4.2 L/100km (56 mpg). Its bigger sibling the 2017 Prius is rated 4.5 L/100km (52 mpg).

So, depending on your needs, I would lean toward the most fuel efficient vehicle to reduce the greenhouse gas emissions and save the most on an ongoing basis.

The Chevy Volt wins if you can plug it in, because it becomes an effective 2.2 L/100km (106 mpg equivalent). If you can't plug it in, then it's a fairly efficient (but not the most efficient) gasoline vehicle.

In regards to the "rated" values above: my Prius V is rated at 5.7 L/100km (41 mpg). The actual measured over the past year is 6.049 L/100km (38.9 mpg). The thing is, the Prius inaccurately displays its average as 5.8 L/100km. This annoys me since it's often up to 0.4 L/100km off per fill-up. That's why I'd use the ratings to compare vehicles, but accept that it's not necessarily indicative of the actual fuel economy you'll experience.