The 2015 Ford Mustang has attracted plenty of attention for its 2.3-liter turbocharged EcoBoost four-cylinder engine--and not all of it positive.

Fans of chest-thumping V-8s may scoff at such a downsized engine, but everyone else will likely want to know how efficient it is.

While not strictly for fuel efficiency, the EcoBoost makes for less-inefficient performance.

DON'T MISS: 2015 Ford Mustang Revealed, With First 4-Cylinder Since 1993 (Photos)

Now, there are some gas-mileage numbers to supplement the speculation.

A rating of 26 mpg combined (22 mpg city, 31 mpg highway) was released for the 2015 Mustang EcoBoost equipped with the six-speed manual transmission.

This fuel-economy data comes from a leaked 2015 Mustang window sticker posted on enthusiast site Mustang 6G.

2015 Ford Mustang GT

2015 Ford Mustang GT

The leak did not include ratings for the six-speed automatic EcoBoost, but it also had gas-mileage ratings for the 3.7-liter V-6.

If the window stickers are genuine, that engine will be rated at 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) achieved by the 2014 Mustang V-6 coupe with the manual, and 23 mpg combined (19 mpg city, 31 mpg highway) with the automatic.

ALSO SEE: Ford Mustang To Eventually Get 10-Speed Automatic?

So the EcoBoost four-cylinder is the most fuel-efficient engine offered in the 2015 Mustang; given its small displacement, that shouldn't be surprising.

However, there is another possible comparison.

Ford offered a turbocharged four-cylinder engine in the Mustang from 1979 to 1993 that--coincidentally--displaced 2.3 liters.

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

2015 Ford Mustang GT

2015 Ford Mustang GT

So how do the two stack up?

With a three-speed automatic, the 1984 Mustang was rated at 20 mpg combined (18 mpg city, 22 mpg highway). That improved to 22 mpg combined (19 mpg city, 22 mpg highway) with the available four-speed manual.

Ford also offered a performance-oriented SVO model at that the time, also a turbocharged 2.3-liter four, that was rated at a mere 19 mpg combined (17 mpg city, 24 mpg highway) with the only available transmission, a five-speed manual.

DON'T MISS: 2015 Ford Mustang EcoBoost: Huge Push For Turbo Four-Cylinder On The Way

Those years of technological improvement make the 2.3-liter EcoBoost 37 percent more efficient than its predecessor, but the true sign of the times is that it produces half again as much power.

When it was introduced in 1984, the SVO turbo four was rated at 175 horsepower, rising to 200 hp by 1986. (The non-turbo four-cylinder produced 105 horsepower as late as 1993 models.)

The EcoBoost is expected to produce 305 hp 310 hp. More fuel-efficient and more powerful: That's progress.

[UPDATE: The original version of this article stated the 2015 Ford Mustang EcoBoost would produce an estimated 305 hp. It will actually produce 310 hp.]


Follow GreenCarReports on Facebook, Twitter, and Google+.