The search engine said it will highlight journeys that generate the lowest carbon footprint using mainly traffic data and road inclines.
Google said the feature would launch first in the US later this year "with a global expansion on the way".
"This is a great example of three trends coming together - the data, sustainability and consumer choice," said Siddharth Pathak, a partner at consultancy firm Kearney. "It will also push those on the fence to make a deliberate choice of speed over sustainability and often cost."