Back in 2009, Ford introduced what would become the backbone of its fuel-efficiency strategy: the EcoBoost line of efficiency-focused engines.

Ford has used the smaller turbocharged, direct-injected engines in most of its models to improve its EPA fuel-economy ratings--even if high gas-mileage numbers aren't always reflected in all buyers' real-world experiences.

RELATED: Ford Cuts Gas Mileage On 6 Models: What You Need To Know, And Open Questions (Jun 2014)

Despite continuing questions real-world versus rated fuel economy, Ford has stuck with the EcoBoost formula for six years, offering more configurations in a growing number of vehicles.

The carmaker recently hit a significant milestone when the 5-millionth EcoBoost-powered vehicle rolled off an assembly line in Wayne, Michigan.

2015 Ford Focus

2015 Ford Focus

That 2015 Ford Focus was one equipped with a turbocharged 1.0-liter three-cylinder--the smallest among the EcoBoost range.

EcoBoost engines rely on turbocharging and direct injection to extract more power from a given amount of displacement, and Ford has used that combination to downsize engines across its model line.

The first EcoBoost engine was a 3.5-liter turbocharged V-6 introduced on the 2010 Ford Taurus SHO performance sedan.

DON'T MISS: 2014 Ford Fiesta EcoBoost: Gas Mileage Test Returns 40 MPG

The EcoBoost lineup has since expanded to include the 1.0-liter three-cylinder, as well as 1.5-, 1.6-, 2.0-, and 2.3-liter four-cylinder engines, and 2.7- and 3.5-liter V-6s.

For the 2015 model year, EcoBoost engines are now available on all Ford cars, crossovers, and SUVs sold in the U.S., as well as most Lincoln models.

Significantly, they're also offered on Ford's best-selling vehicle line, the F-150 full-size pickup truck.

2015 Ford F-150

2015 Ford F-150

All of those new models have had an obvious impact on EcoBoost production. It took Ford less than two years to go from a total of 2 million to 5 million EcoBoost engines.

Yet while the engines continue to proliferate--alongside much smaller numbers of hybrid and plug-in hybrid models--Ford faces continuing questions about real-world fuel economy.

Searching owner forums and reviews from widely varying sources will produce any number of variations on the received wisdom: "You can have Eco or you can have Boost, but you can't have both at the same time."

ALSO SEE: EPA Issues New Guidelines For Automaker Fuel-Economy Testing

Last year, Ford had to lower window-sticker gas mileage ratings on six models, both hybrids and EcoBoost models. The company sent owners "goodwill" checks as compensation for their extra fuel costs.

A 2014 Ford Escape equipped with the 1.6-liter turbo four-cylinder engine also couldn't achieve its EPA-rated figures in our hands during a road test.

So while Ford is offering more fuel-efficient powertrains than ever before, the phrase "your mileage may vary" applies now as much as ever.


Follow GreenCarReports on Facebook, Twitter, and Google+.