One of the most controversial aspects of the all-new 2015 Ford Mustang is its turbocharged four-cylinder EcoBoost engine. The new engine, displacing just 2.3 liters, may upset some traditionalists, but it will provide a more-efficient alternative as Ford Motor Company [NYSE:F] looks to expand the Mustang's appeal both in the U.S. and Europe.

How efficient, exactly? A set of leaked window stickers posted on the Mustang 6G forums provides the first fuel-economy data for the EcoBoost. If the stickers are genuine, the turbo four will be EPA-rated at 26 mpg combined (22 mpg city, 31 mpg highway) with the six-speed manual transmission (figures for the six-speed automatic weren't released).

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Figures were also released for the 3.7-liter V-6, an updated version of the engine offered in the current Mustang. For 2015, it will achieve 21 mpg combined (17 mpg city, 28 mpg highway) with the manual transmission and 22 mpg combined (19 mpg city, 28 mpg highway) with the automatic. That's actually a slight decrease from the 22 mpg combined (19 mpg city, 29 mpg highway) rating for the 2014 Mustang V-6 coupe manual, and 23 mpg combined (19 mpg city, 31 mpg highway) with the automatic.

The EcoBoost beats both of them but, given its smaller displacement, that's not too surprising. Of course, this isn't the first time Ford has offered a four-cylinder engine in the Mustang. A four-cylinder that—coincidentally—displaced 2.3-liters was available in the Mustang from 1979 to 1993, so how does it stack up to the EcoBoost?

Fuel economy data for the 1984 model is available on the EPA website, adjusted to revised rules introduced in 2007. Thirty-one model years removed from the 2015 Mustang, it's the earliest model available for comparison.

The base 1984 Mustang was rated at 20 mpg combined (18 mpg city, 22 mpg highway) with a three-speed automatic, and 22 mpg combined (19 mpg city, 22 mpg highway) with the available four-speed manual.

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Ford also offered a sporty SVO model at the time with a more-powerful turbocharged 2.3-liter four. It was rated at just 19 mpg combined (17 mpg city, 24 mpg highway) with the only available transmission, a five-speed manual.

Three decades of technological advancement doesn't bring a dramatic increase in efficiency, then, but that's not the only way to measure progress.

In 1993, its last year, the base, non-turbo four-cylinder engine was rated at 105 horsepower, while the SVO produced 200 hp in its last year, 1986. The 2015 EcoBoost, on the other hand, is expected to produce 310 hp. Now that's progress.

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