Volkswagen admitted in late September that it had been gaming EPA tests by using an “emissions defeat device” to manipulate the nitrogen oxides (NOx) output of its diesel vehicles.

The vehicles’ software was programmed to pass emissions testing performed by the EPA. But once on the open road, the cars would pollute NOx at up to 40 times the federal maximum. According to Volkswagen, 11 million cars are affected worldwide.

EPA tests both emissions and fuel economy on a dynamometer, which this emissions defeat device was designed to detect. Emissions were suppressed in this “cheat mode”—but fuel economy was too. However, once the cars were no longer on the dynamometer, the cheat mode turned off. That meant that in real world driving conditions, the diesel cars not only started polluting again, but they also could get better fuel economy.

Volkswagen has tried to portray the emissions-cheating software as a mistake that slipped through the corporate cracks—laying much of the blame on some rogue engineers.

But the company was happy to brag in its publicity efforts about the real-world fuel-economy benefits, which appear to be a result of the software manipulation.

For instance, in a 2008 press release, VW touted and quantified how the car would perform differently on the road than in the laboratory. Here's an excerpt:

“While the Environmental Protection Agency estimates the Jetta TDI at an economical 29 mpg city and 40 mpg highway, Volkswagen went a step further to show real world fuel economy of the Jetta TDI. Leading third-party certifier, AMCI, tested the Jetta TDI and found it performed 24 percent better in real world conditions, achieving 38 mpg in the city and 44 mpg on the highway.” 

Consumer Reports also experienced impressive fuel economy and recommended these vehicles at the time. (We have suspended our recommendations of the Jetta diesel and Passat diesel until we can re-test them after VW has made them compliant with emissions regulations.)

We even calculated how the premium consumers paid for VW’s diesel engines would pay for itself in time. What we didn’t know was that the emissions control software had been manipulated.

Those EPA-beating, on-the-road fuel economy numbers were part of the reason many people bought these cars in the first place. Many vehicles underperform the EPA fuel economy window sticker, so the fact that VW diesels did better than the EPA test results was a significant marketing asset for the automaker.

Once it came to light that VW’s cheat mode bypassed proper emissions when in the real world, Consumer Reports tested a 2011 Jetta Sportwagen TDI to check the difference. We tricked the vehicle’s software into thinking it was being tested by the EPA in a lab, and we indeed saw slightly slower acceleration and a 4-mile-per-gallon degradation in fuel economy on our highway test circuit.

In Congressional testimony, Volkswagen’s U.S. CEO Michael Horn said that, after the VW diesels are fixed to become fully emissions compliant, they will still meet “sticker fuel economy." But Horn did not elaborate as to whether fuel economy would decline. Since Volkswagen knows their cars currently get better than EPA-rated fuel economy, meeting sticker fuel economy still allows for a MPG drop.

To see if beating EPA estimates is a broader diesel trend, we looked at the 16 diesel cars we tested over the last five years. In the Consumer Reports tests, the only vehicles that beat the EPA fuel economy estimates were Volkswagens. Note, this includes the 2009-2012 V6 Touareg. VW had stated it expected fuel economy higher than EPA estimates when it introduced the larger V6 alongside the smaller recalled engines.

Volkswagen has made diesels a key part of their growth ambitions in the U.S. market. And the strategy was working—VW carries a 75 percent market share of diesel-engine passenger cars. It seems highly unlikely that Volkswagen created its emissions defeat device just to outperform EPA mileage estimates, but the company sure used the artificially high numbers that resulted from the cheat to try and promote these cars.

We are sending details of our historical diesel fuel-economy analysis to the EPA as it may help their ongoing investigation. Once VW's fix is in place, CR will re-test our VW diesel vehicles to see how significant the performance and fuel economy degradation is to consumers.