EPA claims its hybrid vehicle tests remain accurate, but says Ford C-Max Hybrid exploited a loophole.

The EPA says it stands behind its fuel economy test for hybrid vehicles following controversy about the testing process after Ford C-Max Hybrid customers and automotive journalists alike struggled to achieve 47 miles per gallon, the advertised mpg number, Automotive News reports. Ford responded to the issue almost two weeks ago by claiming that a 1970s-era EPA general label rule was responsible for the inaccurate mileage numbers, rerating the C-Max Hybrid's mpg numbers and offering customers rebates. Ford later said it didn't overstate the C-Max Hybrid's fuel economy and that it was surprised by the low numbers.

Ford technically didn't do anything wrong because it was following the general label rule, but agency regulator Christopher Grundler says the automaker was exploiting a loophole when it came up with the hybrid C-Max numbers, and that the testing process remains accurate. The general label rule allows vehicles that use the same engine and transmission and are in the same weight class to share fuel economy numbers, but it doesn't take into account other factors such as aerodynamic efficiency, which affects hybrids more drastically than non-hybrid vehicles. Ford originally used the Fusion Hybrid economy figures for the C-Max Hybrid and claimed the engineers didn't realize that its aerodynamic efficiency would affect fuel economy as much as it did.

To address the increasingly commonplace practice of using the same powertrain in multiple applications, Grundler says, the EPA rules need to be changed so more car buyers are not mislead.

Toyota is standing with the EPA on this one after spurring a revision of test methods in 2006 after overstating Prius mileage numbers. Toyota said in a statement that it believes "the current labeling methodology established since 2006 provides appropriate fuel economy label values for customers, when automakers apply these rules with good common sense and engineering judgement." And yes, that sounds like a well-aimed but somewhat ill-timed shot from Toyota to Ford to us, too...