I thought this was a pretty good article by Popular Mechanics:
http://www.popularmechanics.com/cars/news/fuel-economy/mileage-moment-of-truth-we-put-40-mpg-claims-to-the-test-6651300-2
If the Elantra is getting 40+, I'm sure all the other 40mpg products in Hyundai's lineup are getting similar...