How much acceleration and how many MPGs would Volkswagen’s cars really lose without their infamous cheat code? The vehicle-testing gurus at Consumer Reports are here to answer that, and for owners, the answer is not pretty.

We’ve already shown you TFLcar’s Volkswagen Diesel test where they claimed a performance drop in up to 15 horsepower and 32 lb-ft of torque.

While TFLcar’s test drew quite a bit of skepticism for its questionable test setup, Consumer Reports is known for conducting in-depth, scientific vehicle tests. That’s what they do. That’s why people read their stuff.

So Consumer Reports decided to put a 2015 Volkswagen Jetta TDI and a 2011 Jetta Sportwagen TDI to the test. To activate the car’s cheat mode, they pumped the accelerator pedal 5 times while the ignition was in the “on position” but the engine was off. To avoid being “kicked out” of the cheat mode when the car noticed the rear, undriven wheels spinning, CR simply unplugged the rear wheel speed sensors. Simple as that. Cheat mode activated.

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They ran their standard Consumer Reports drive cycle. This involved a city traffic simulation on the track and a 65 MPH highway run on the local highway.

Fuel economy numbers were determined by taking fuel readings from a flow meter and averaging MPG numbers in both directions to account for grades and wind speed, and tests were run several times with different drivers to guarantee consistency.

Here are the numbers: On the 2011 Jetta TDI, 0-60 times increased from 9.9 to 10.5 seconds but the 2015 Jetta’s acceleration numbers were unaltered.

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But the biggest changes occurred in fuel economy. Especially on the highway where the 2015 Jetta’s MPGs dropped from 53 to 50 and the 2011 Sportwagen’s plummeted from 50 to 46 MPG.

That’s a pretty significant change for both cars, and considering how many different cycles Consumer Reports ran and how many checks they have for verifying their figures, these numbers definitely seem credible.

But Volkswagen’s fix probably won’t involve always running with the “cheat mode” engaged— there may be durability concerns. We don’t know yet. So TDI owners— we’re not saying your car will become lethargic and lose about 10% in fuel economy. We really won’t know until VW decides how they’ll fix their cars.

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