The compression-ignition gas engine’s optimistic Euro MPG numbers could still translate to huge EPA estimates in the U.S. Mazda is officially putting its advanced Skyactiv-X gasoline engine on sale—at least in Europe. The engine is coming to the U.S. market later. The automaker has also made available the first official specifications and fuel-economy data for the new engine, though they’re only applicable to Europe for now. Until this point, we’ve had only estimates for the X’s horsepower and torque from when we drove a Mazda 3 with a prototype version of the engine, along with vague hints of big-time efficiency. So, here are the details: In 2.0-liter form, the Europe-market Skyactiv-X engine produces 178 horsepower at 6000 rpm and 165 pound-feet of torque at 3000 rpm. Those figures are slightly lower than those of the U.S-market Mazda 3′s 2.5-liter four-cylinder, which makes 186 horsepower (also at 6000 rpm) and 186 lb-ft of torque (at a higher 4000 rpm). The Sky-X engine has an incredibly high 16.3:1 compression ratio, though actual cylinder pressure varies. Most impressively, the Mazda 3 equipped with the Skyactiv-X 2.0-liter engine gets 50 mpg...