Fuel economy numbers are critical to selling cars. They get trumpeted in advertising and many people base their car buying decisions on them. The results are so important that car companies go to great lengths to “hack” fuel economy tests in the pursuit of better numbers, which is a major reason why those of us out in the real world seldom get the kind of gas mileage the official tests lead us to expect.

In Europe, official fuel economy ratings are about 30% higher than real-world fuel economy says Transport Environment, a sustainability advocacy group. That costs a typical driver in Europe more than $600 a year in fuel costs they wouldn’t have to pay if the advertised numbers were accurate. So what gives?

Mercedes-Benz is one of the most aggressive car makers when it comes to cooking the fuel economy books, but other manufacturers are not that far behind. The gamesmanship begins with the vehicle an automaker submits for testing. Often, it is a “specially prepared prototype.” What does that mean?

To start with, low rolling resistance tires are fitted and then overinflated until they are rock hard. Then suspension settings are reset from factory specs to minimize friction losses. Engineers sometimes fit special brakes are that retract the pad material as far as possible to eliminate even more friction. Driving a car set up this way on public roads would actually be dangerous, yet these nearly imperceptible changes make a big difference for MPG ratings. But the cheating isn’t over.

Some automakers even disable the alternator to eliminate parastic drag on the engine. Special lubricants are used within the engine to reduce friction, and finally, all the gaps between the body, doors, and hood are taped shut to eliminate air turbulence. Is that it?

Sadly, no, as sometimes engineers do some actual hacking to get the numbers they want. Sophisticated programming of the car’s engine management system can detect when a test cycle is underway. When that happens, the computer tells the transmission to select higher gears than normal and dials back engine performance to reduce the amount of fuel consumed to a minimum, boosting even fuel efficient models well beyond the scope of what drivers should actually expect. For example, the Toyota Prius rates at 50 MPG in the U.S. testing method, but over 62 MPG in Europe, numbers you’ll only see if you never go above 50 MPH. No wonder we seldom get the kind of miles per gallon in our own cars we expected.

Europe hopes to deal with test cheating with its new World Light Vehicle Test Procedure due in 2017. But don’t hold your breath. Auto makers are working feverishly behind the scenes to delay the new test protocols until 2022.

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Tags: cheating on fuel economy tests, engine management system, fuel economy testing, low rolling resistance tires, special lubricants

About the Author

Steve Hanley I have been a car nut since the days when articles by John R. Bond and Henry N. Manney, III graced the pages of Road Track. I know every nut, bolt and bullet connector on an MGB from 20 years of ownership. I now drive a 94 Miata for fun and the occasional HPDE track day. If it moves on wheels, I am interested in it. Please follow me on Google + and Twitter.

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