Lately, automakers haven't been touting the rising fuel economy of new vehicles, but it has to continue rising to meet reductions in the legal levels of carbon emitted.
While major strides in fuel economy have been made over the past five years, the complex engine technologies and other improvements turn out to have less effect in real-world driving than the numbers would indicate.
In layman's terms, a tiny turbocharged inline-4 cylinder engine may not be as efficient as lab results suggest.
Those findings come from the latest round of intensive research undertaken by UK-based Emissions Analytics, which has tested more than 500 cars in real-world driving situations in the United States.
Globally, it's now tested more than 1,000, and the results may be discouraging for many automakers, according to a preview of the research from Automotive News (subscription required).
EA's real-world testing shows no actual overall improvement to fuel economy and no decrease in CO2 emissions over earlier vehicles of the same size, despite increasing numbers of new technologies and more complex powertrains on new vehicles.
2018 Chevrolet Malibu
EA compiled its data using an 88-mile course in Southern California, where its equipment measured actual tailpipe emissions of carbon dioxide, nitrogen oxides, and other pollutants, along with the actual fuel economy.
Its results suggest three major takeaways.
Engines of less than 2 liters have seen zero changes in fuel economy; that for engines displacing 2 to 3 liters has actually decreased around 8 percent; and only engines larger than 3 liters have increased their fuel economy, about 8 percent.
EA believes the efficiency decrease in the most common engines in the U.S., the 2- to 3-liter units, has to do with longer commute times and the types of vehicles the engines are powering.
The engines may be small and efficient, but they lose those characteristics when drivers cover longer distances in larger, heavier crossover utility vehicles, rather than smaller, lighter compact sedans.
The research also found turbocharged engines are some of the worst offenders in failing to keep their efficiency promises.
2015 Ford Focus SE EcoBoost, Bear Mountain State Park, NY, May 2015
In EPA emission testing in laboratory settings, the engines are under low stress and produce excellent fuel economy figures.
In the real world, when the turbos are actually used frequently to keep up with traffic far more aggressive than the test cycles, the engines actually proved less efficient than a naturally-aspirated engine of the same power.
Some good news arrived on the hybrid front, however: EA found traditional hybrids provided excellent efficiency under real-world driving conditions, and kept CO2 levels low as well.
Automakers that have reduced the weight of their vehicles, employed more gears in their automatic transmissions, and used higher-efficiency tires also scored well in EA's real-world fuel economy tests.
The full report is due to be released in October of this year, and the company will provide more specifics on which technologies prove to help improve efficiency.
It will also highlight those technologies, likely include many turbocharger applications, that consumers should avoid because they provide higher EPA fuel-economy ratings but can't deliver on them in actual use.