Just found this "Skyactiv-G engines for the U.S. market have a lower compression ratio of 13:1 allowing them to operate on standard instead of premium fuel with an approximate 3-5 percent reduction in torque and fuel economy."
Source: "Mazda Skyactiv-G and Skyactiv-D Engines in Detail". Caranddriver.com. Retrieved 13 July 2018. https://en.wikipedia.org/wiki/SkyActiv
"Premium Fuel, Mid-Grade Output
Premium, 91-octane fuel is required for the Sky's not-so-staggering 163 hp at 6000 rpm and 155 lb-ft at 4000, but Mazda is proud of its exceptionally wide torque band for enhanced real-world drivability. To enable running on regular gas, the U.S. version will have a compression ratio of 13:1, which means fuel economy and torque will diminish by about 3 to 5 percent, according to Mazda. The premium-fueled Sky we drove was perfectly adequate in the Mazda 6 prototypes, although acceleration was rather leisurely—far slower than the current Mazda 6 with its 168-hp, 2.5-liter—giving us plenty of time to wish for a bit more smoothness during the extended time in each gear. But being in the lighter Mazda 3 would help, and the tradeoff for near-diesel levels of fuel economy is probably worth it."
Source: "Mazda Skyactiv-G and Skyactiv-D Engines in Detail". Caranddriver.com. Retrieved 13 July 2018. https://en.wikipedia.org/wiki/SkyActiv
"Premium Fuel, Mid-Grade Output
Premium, 91-octane fuel is required for the Sky's not-so-staggering 163 hp at 6000 rpm and 155 lb-ft at 4000, but Mazda is proud of its exceptionally wide torque band for enhanced real-world drivability. To enable running on regular gas, the U.S. version will have a compression ratio of 13:1, which means fuel economy and torque will diminish by about 3 to 5 percent, according to Mazda. The premium-fueled Sky we drove was perfectly adequate in the Mazda 6 prototypes, although acceleration was rather leisurely—far slower than the current Mazda 6 with its 168-hp, 2.5-liter—giving us plenty of time to wish for a bit more smoothness during the extended time in each gear. But being in the lighter Mazda 3 would help, and the tradeoff for near-diesel levels of fuel economy is probably worth it."