Last year, BMW introduced a new eight-speed automatic transmission plus a new turbocharged four-cylinder engine in the base 328i Sedan models, which gave BMW 3-Series a big boost in fuel efficiency. Those efficient powertrains continue into 2013, but this year there's also a new ActiveHybrid3 model.
Unfortunately, with a turbo six instead of a four-cylinder engine under the hood, the ActiveHybrid3 likely won't much more fuel-efficient than the 328i and 335i models—both earning up to 23 mpg city and 33 or 34 mpg highway. But with a supplemental electric-motor system and 675Wh of lithium-ion cells in the trunk, it boasts an all-electric range of 2.5 miles—a feature that could boast real-world mileage in stop-and-go driving significantly, if you can show some restraint. The system can also shut off the gasoline engine when coasting at high speeds (up to 100 mph).
All BMW 3-Series are equipped with a stop-start system that shuts off the engine for long periods, such as at a stoplight. A brake energy regeneration system is standard on all models too; it keeps the alternator from drawing a heavy load from the engine until the brakes are engaged, to keep the engine running as efficiently as possible during normal circumstances. The air conditioning compressor runs more efficiently, and there's an Eco Pro mode that keeps auxiliary controls running as lean as possible to eke out every mpg from every drop of gas.
The Auto Start-Stop system can be pretty rough with the six-cylinder—delivering more of a shudder/kick than we've felt from other systems—but it's smoother with the four.
Going turbocharged has a very positive effect on fuel economy. BMW 328i models with the automatic earn 34 mpg on the highway, which is on par with some four-cylinder mid-size sedans that aren't nearly as exciting to drive.