Fuel Comsumption vs. Emissions

I've been wondering... My assumption has always been that the vehicle that can go the farthest on a set amount of fuel is the most environmentally friendly. Is that a correct assumption? Do some vehicles go farther but produce more harmful by-products? But then thinking over the course of lifetime use, does the saved fuel of the more "efficient" vehicle offset the more harmful emissions of the other...? Thoughts?