Much better than any mid-size in the past—think back to the drinky turbo T-Types—today's Buick Regal sedan doesn't quite reach the gas-mileage heights of larger, mass-market four-doors.
Last year's base inline-4 engine is gone, which leaves the eAssist mild-hybrid drivetrain as the starting point to get into any Regal. Even though it has hybrid running gear and the EPA officially calls it a hybrid, GM dials down those claims by pointing out the eAssist is a milder version of electrification. The Regal uses lithium-ion batteries to boost acceleration under certain circumstances, and to help run power-sapping accessories, but the Regal can't run on electrons alone, which is different than most other hybrids.
The EPA rates the eAssist 4-cylinder and 6-speed automatic combo at 25 mpg city, 36 highway. That's not much better than the base Hyundai Sonata/Kia Optima, nor the Ford Fusion with a 1.6-liter EcoBoost. The Regal's numbers fall behind the 38-mpg highway figure set by the Nissan Altima too.
Mileage in the turbocharged Regal drops too, either with a manual or an automatic transmission. The 2.0-liter turbo-4 in the Regal is rated at 20/32 mpg with the manual or 18/29 with the automatic. Opting for more power in the Regal GS lowers those numbers too, at 19/27 mpg for the manual and automatic versions.