"A group of scientists has named the top green automaker, and it's not Toyota." That's how The Nature Conservancy's Brad Parker announces the news. He's not alone among environmentalists who are taken aback. "This might come as a surprise to Prius owners, whose image of Toyota is tinted green," says the Los Angeles Times' Geoff Mohan. For the fifth time in a row, Honda took the Union of Concerned Scientists' top spot. Here's why, according to their report summary:

Sales matter. Delivering an environmentally friendly fleet requires producing clean vehicles and selling them. This fact has important implications for existing hybrids, as well as for upcoming plug-in hybrids and electric vehicles. Moreover, while such technologies are both promising and inspirational, automakers seeking to make good on claims of eco-stewardship need to focus on cleaning up their entire fleet, including their biggest sellers.
So Honda may not have the best hybrid design, but its "historic leadership in fuel economy" across the board seems to have pushed it ahead. Toyota, on the other hand, relies almost entirely on hybrids for its "eco-competitiveness." In fact, without the hybrids, "the company would have finished in fifth place. For Toyota to claim the mantle, it must maintain the lead it holds in hybrid technology while also improving its conventional vehicles."