The errors — covering 41% of the vehicles sold by the two companies since 2010 — are the largest ever uncovered by the U.S. Environmental Protection Agency since the federal government first set fuel economy standards in 1975. The agency says the errors were unearthed after EPA officials couldn't replicate the mileages Hyundai and Kia had claimed on window stickers, which the companies will have to replace on new models sitting on dealers' lots.

Under the corrections announced today, Hyundai now has no models that top 40 mpg.
Hyundai and Kia say the errors were due to a flaw resulting from changes to its fuel economy tests starting in the 2010 model year that overstated efficiency on 13 models, including popular sellers such as the Hyundai Sonata and Accent. While the errors were often one to two miles per gallon high, in some vehicles they were far more; the 2011 Kia Soul 2-liter automatic's rating of 26 mpg city and 34 mpg highway has been corrected to 23 mpg city and 28 mpg highway.
The affected models include the 2011 Hyundai Elantra, Hyundai Sonata Hybrid and Kia Optima Hybrid; the 2012 and 2013 Hyundai Accent, Azera, Elantra, Genesis, Sonata Hybrid, Tucson and Veloster; the 2012 and 2013 Kia Rio, Sorento, Soul and Sportage; and the 2013 Hyundai Santa Fe.