How much performance do Volkswagen four-cylinder diesel engines lose when in the special "cheat mode" that the company has admitting it used to sidestep U.S. emissions tests? Consumer Reports decided to find out with a series of tests it conducted in this new video report.

Installed in 11 million Volkswagen and Audi diesel models worldwide, Volkswagen's special software programming is designed to run the 2.0-liter four-cylinder diesel engine in a more restrictive emissions control mode when the cars are being emissions tested. Presumably this mode offers reduced power and fuel economy performance than the cars manifest when driven in normal conditions, when the "defeat device" installed in the cars turns off full emissions control.

To perform a direct comparison between these two driving modes, CR figured out a way to put two VW diesels, a 2015 Volkswagen Jetta TDI and a 2011 Volkswagen Jetta SportWagen TDI, into the cheat mode. The first step is to deactivate the traction control and stability control systems. To keep the car thinking that it is on a dynamometer, CR then disconnected the rear wheel sensors so the car could not tell that the rear wheels were moving.

The results showed a distinct difference between the two modes in both vehicles. In the 2011 Jetta, acceleration slowed by half a second, while the 2015 Jetta only dropped 0.1 second. Fuel economy tests showed a more significant disparity, with the 2015 Volkswagen Jetta dropping from 53 mpg to 50 mpg and the 2011 Volkswagen Jetta dropping from 50 mpg to 46 mpg on CR's highway test.

This loss in performance could have implications for how Volkswagen plans to fix the millions of cars that are equipped with this software. A simple software update would likely result in a similar performance degradation as shown in these tests. Other possibilities include the addition of a urea filtration system to restrict emissions, which would be expensive and time-intensive.

Stay tuned to hear more about how Volkswagen plans to fix the affected vehicles, and take a look at the video below to see the VW's infamous "cheat mode" in action.