With the success of the Toyota (TM) Prius and the mad dash into sustainable vehicle development, wearing a green mantle has become a key marketing tool for major automotive brands. As a result, the Greenest Car rankings, released on Sept. 24 by Whatgreencar.com, are getting a lot of attention. The site tabulated rankings based on several key data points including fuel efficiency and emissions.

The two big winners were Chevrolet and GMC, traditionally among the laggards due to their past heavy reliance on big trucks and big engines. According to the findings, Chevy managed to improve the environmental friendliness of its 2010 lineup by 20.3 percent and GM managed an impressive 15.3 percent improvement. Eco-icon Toyota, with its vaunted Prius hybrid, didn't even make the top 10. Go figure.

Overall, the 2010 fleet showed an average eco-improvements of 6.6 percent year-over-year, a strong showing considering that the previous year's improvement was only 2.1 percent. Other car makers rounding out the Top 10 improvement range included: Mercedes-Benz (13.6 percent), Lexus (13.2), Mercury (11.6), Kia (11.0), Ford (10.4), Acura (8.0), Volkswagen (8.0), and Suzuki (7.7). Aside from Toyota, another notable environmentally friendly carmaker, Honda Motors (HMC), was also missing from the list, although the company's higher-end (and more gas guzzling) Acura line did make the list.

So why did Toyota and Honda fail to make an appearance? It's possible that these two brands have already greened themselves so much that improvements are difficult. Alternatively, they may have had few or no product refreshes. That could also help explain why Chevy and GM did so well -- it's easier to improve a gas guzzler than a sipper. Ironically, GMC and Mercedes both made the Top 10 Most Polluting Fleet list as well, joining the ranks of Hummer and Lamborghini, suggesting there is still plenty of room for improvement.

Alex Salkever is Senior Writer at AOL Daily Finance covering greentech and environmental issues. Follow him on twitter @alexsalkever, read his articles, or follow our green articles twitter feed (@dfgreentech).