Today the U.S. Environmental Protection Agency said it would begin on-road emissions testing of new and used vehicles.

The changes come on the heels of a similar move in Europe in response to Volkswagen’s disclosure it had installed software to detect emissions testing and put the car in a model to let them pass.

EPA sent a letter to automakers informing them of the sweeping changes. It says it will start testing light-duty vehicles already on American roads to check for potentially out-of-compliance cars.

“EPA may test or require testing on any vehicle at a designated location, using driving cycles and conditions that may reasonably be expected to be encountered in normal operation and use, for the purposes of investigating a potential defeat device,” EPA told automakers in the letter.

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The EPA said this could slow down the certification process of cars for sale in the U.S., and is putting others on notice. It also is specifically on the look-out for defeat devices such as VW utilized in 11 million cars globally, it said to reporters today.

“We aren’t going to tell them what these tests are,” said Christopher Grundler, director of the EPA Office of Transportation and Air Quality, in a conference call with reporters. “They don’t need to know.”

Yesterday also we reported Europe is doing the same, with “real driving emissions” (RDE) testing to be phased in this decade, and it has been Europe where existing regulations were seen as easier to look good on.

Compared the U.S. European regulations have given a cleaner emission and mpg rating to vehicles. Automakers have used these higher numbers accordingly to market their cars.

Today’s move is mainly reactive to close potential loopholes in existing testing.

The old adage “your mileage may vary” remains in full effect.