In the August, 2014 issue MT published an investigation as to how different cars' mpg measured up to the EPA ratings. They used a very sophisticated analysis methodology, but of course I have no idea how accurate it was. Overall, they found that diesel engines underperformed the EPA numbers by 3.4%, which was much higher than any other kind of car. For instance, hybrids came in at only 0.5% over the EPA ratings.



You can go here to see specific cars - http://www.intellichoice.com/real-mpg.html (Elantra, Santa Fe, and Sonata only listed for Hyundais)