Replace big, dumb engines with small, smart ones that – thanks to the tireless march of technology – generate the same level of power.

Ford has been at the fore of this phenomenon, relying on turbocharging to squeeze big horsepower from small power plants, and much of the industry is lining up to follow. The only wrinkle: early adopters are finding that though engines have shrunk, promises of boffo fuel economy have been grossly inflated.

On web forums and in letters to manufacturers, consumers have expressed surprise and dismay at their cars’ not living up to their fuel-economy estimates.

Editors at Consumer Reports have echoed those grievances. In their testing of a 2013 Ford Fusion sedan equipped with the brand’s 1.6-litre EcoBoost 4-cylinder engine, they found that the car was slower accelerating from a stop to 60mph and returned lower gas mileage than competitors powered by larger, non-turbocharged engines.

The Chevrolet Cruze compact sedan offers a direct comparison, as the car is available with turbocharged and normally aspirated engines, both rated with the same horsepower. Consumer Reports found the old-fashioned 1.8-litre engine was nearly as quick to 60mph as the turbocharged 1.4-litre unit, and returned the same real-world gas mileage but without the added cost and complication of turbocharging. (The publication does not release its test methodology, so it is impossible to independently verify its findings.)

“Turbochargers pump more air into the engine during strategic times so it can burn more fuel and make more power during peak demand periods such as passing,” said Drew Winter, editor in chief of WardsAuto World, a trade magazine that issues an annual list of its 10 best engines, in an email to BBC Autos. “When the engine does not need power, the [engine downsizing strategy] does work and the engine behaves like a smaller engine and burns less fuel.”

In practice, however, things are turning out differently. “On most turbo engines you need to have a very light foot to stay out of the turbo boost,” Winter said. “With some of the smallest turbo engines, there is a tendency to get into the throttle more, which uses more turbo boost and can result in worse fuel economy than a larger standard engine.”

Invited by Ford to co-drive in the company’s record-setting test last year of its 1-litre 3-cylinder EcoBoost engine for 24 hours, I asked an engineer on hand about fuel consumption over the course of the run, during which the car I co-drove, a Focus hatchback, averaged 107mph. The engineer said the fuel economy was worse than if we had done the run using the company’s larger, conventional 1.6-litre 4-cylinder engine because smaller, turbocharged engines respond to strain by burning more fuel. If we had driven more slowly, the EcoBoost would have bettered the bigger engine, he explained, but at full speed it was mighty thirsty.

The same effect is observable among powerful sporty cars. The snorting, V8-powered 2012 Chevrolet Corvette returned 21mpg in Consumer Reports’ testing – superior to the figure achieved by the 4-cylinder Mitsubishi Lancer Ralliart (20mpg) and tying the Volvo C70 T5 5-cylinder coupe.

Another knock against the boosted little engines is that they lack the refinement of larger units with more cylinders. Consumers appreciate the smoothness of 6-cylinder engines as much as their power. Turbocharged 4-cylinder engines may match V6s in power, but not in refinement, Consumer Reports said this week in their statement on the matter.

The turbo engines do enjoy strong mid-range power, the editors acknowledged. “In daily driving this means a more effortless feeling of thrust with reduced need to downshift while climbing hills or when delivering the kind of moderate acceleration most drivers demand,” they said. “That can make a car feel more responsive, even if its actual acceleration times from a standstill are slower.”

Judging from the fuel economy drivers are reporting, their acceleration in the real world may not be slower. Rather, they may just be standing on the gas more heavily to keep pace with traffic, which is no way to achieve boast-worthy mileage.

How dumb do those big engines look now?