The 2012 BMW 335i sport sedan holds the line against the weight gain that detracted from some of the automaker’s recent vehicles. At $54,850 for the loaded car I tested, the 335i’s price tag is awful hefty, though.

The 2012 3-series sedan is all new, despite styling that’s evolutionary in the sense that living creatures evolve over millennia, with virtually no discernible difference from one generation to the next. Unless you’re a BMW salesman, you may have a hard time recognizing the 2012 335i.

Under the skin, the differences are profound and for the good. The new car has more passenger and luggage room, a more powerful base engine and better fuel economy.

The 3-series is BMW’s best-selling vehicle. It comes in a wide array of models, but only the sedan is new for 2012. New versions of the coupe, convertible and station wagon are coming, but all BMW will say about timing is not to expect any this year.

A hybrid arrives this fall, and an optional all-wheel drive model. It’s unclear whether BMW will continue to offer a diesel.

Prices for the 2012 BMW 3-series sedan start at $34,900 for a 328i with a 240-horsepower turbocharged 2.0-liter four-cylinder engine and six-speed manual or eight-speed automatic transmission.

The 2.0-liter base engine is new. It replaces a normally aspirated 3.0-liter straight-six that used more fuel and produced less power. The new 328i is slightly heavier than the 2011 model.

The 335i uses a turbocharged version of the 3.0-liter I-6 that produces 300 horsepower and 300 pound-feet of torque. The 2012 335i is lighter and more fuel efficient than the old model. Prices start at $42,400 for a six-speed manual or eight-speed automatic.

I tested a very well-equipped $54,850 335i. It had the manual transmission and options that included a navigation system, heads-up display, Harman/Kardon sound, 19-inch alloy wheels and adaptive suspension.

The 335i competes with the Acura TL SH-AWD, Audi S4, Cadillac ATS, Infiniti G37, Lexus IS 350 and Mercedes-Benz C350.

The 335i sedan’s base price compares well to all those cars except the ATS, for which prices have not been announced. The 3-series’ cost rises with dismaying speed as you add options though.

The 335i is a delight to drive. It practically leaps to life as the turbocharged straight-six produces 300 pound-feet of torque from just 1,200 rpm. The silky six-speed manual transmission is virtually effortless. The light clutch makes it easy to work the gears enthusiastically.

The suspension holds the 335i firmly planted during hard cornering and absorbs bumps well. The steering is quick and precise. It felt a bit light at 40 to 60 mph, however.

Road and wind noise are loud enough to impede conversations.

Despite being 3.7 inches longer, 0.2 inch wider, 0.4 inch taller, and having more passenger and luggage space, the 2012 335i weighs a few pounds less than a comparably equipped ’11 model.

The 335i I tested rates an excellent EPA combined city/highway fuel economy rating of 23 mpg. Only the C250 Sport can match that, but the Mercedes only comes with an automatic transmission, and a 335i with BMW’s eight-speed automatic trounces it by 3 mpg.

The 3-series has an auto-stop feature that shuts the engine off when the vehicle sits idling. It worked poorly with the automatic transmission in a 528i sedan I tested recently.

It’s a bit better with a manual, but only operates when you shift into neutral and release the clutch. That process masks some of the system’s flaws.

It’s also inconvenient enough that I doubt many drivers will use the feature much.

Stop-start can be responsible for 3 to 4 percent of a vehicle’s EPA combined rating. BMW’s system needs work.

The larger interior increases head and leg room. Big map pockets offer good storage space, but the bin in the center console is small. You’ll be hard put to find a good place for sunglasses, phone and music player.

A 6.5-inch color display and BMW’s continually improving iDrive system make it reasonably easy to control the car’s secondary systems.

The interior materials are appealing. A soft and attractive black padded plastic covers the doors and dash. The steering wheel, shifter and seats all had leather, and the trim made restrained use of brightwork. The surfaces of the doors and dash are cast in sweeping, flowing shapes.

The light, sleek 335i sedan’s comfort, performance and fuel economy set a high standard that goes a long way toward justifying the car’s high price. A quieter interior and better auto-stop would be welcome though.

BMW 335i

Base price: $42,400

As tested: $54,850

Type: Rear-wheel drive five-passenger sport sedan

Engine: 3.0-liter turbocharged direct-injection DOHC variable timing 24-valve straight-six; 300 horsepower at 5,800 rpm; 300 pound-feet of torque at 1,200-5,000 rpm; six-speed manual or eight-speed automatic

Mileage: 20 mpg city, 30 mpg highway