Combining advanced stop/start technology with a previous Ward’s 10 Best Engines winner for a promised highway fuel-economy gain of as much as 32%, the Buick LaCrosse eAssist entered our garage as an early favorite to nab an award.

After 500 miles (805 km) of testing, it left the scene humbled like a fizzled first-round draft pick.

The LaCrosse eAssist generally drew raves from WardsAuto editors for expertly integrating a 2.4L direct-injection 4-cyl. (2010 winner) with an advanced 115V Lithium-ion battery pack and 15-kW (20-hp) electric motor-generator. But it simply came up short in the fuel-economy ledger.

In a mix of real-world city and highway driving, the car turns in an average of less than 24 mpg (9.8 L/100 km), short of GM’s estimated 25 mpg (9.5 L/100 km) city mark.

Its peak during testing was 27.7 mpg (8.5 L/100 km), and the LaCrosse eAssist never approached the 36 mpg (6.5 L/100 km) bogey GM says it can reach on the highway.

The LaCrosse’s numbers around metro Detroit also fall short of the 26.6 mpg (8.8 L/100 km) our editors log during combined-cycle testing of the car earlier this year in California.

Recent testing in Texas of the ’13 Chevy Malibu Eco with eAssist, a car boasting nearly exactly the same specifications as the LaCrosse, yields the same 26.6 mpg in mixed-cycle driving.

“Kudos to GM for stop/start, but 22 mpg isn’t raising the bar,” editor Tom Murphy says after his stint at the wheel of the LaCrosse.

Fellow editor Christie Schweinsberg says after her turn: “Disappointing, in a word.”

The deflating fuel economy comes despite mostly positive praise of the system’s operation, which shuts down the engine at stops. An electric motor helps deliver a smooth restart and occasionally helps propel the vehicle, such as on uphill grades that prove too demanding for LaCrosse models using just the 2.4L I-4.

GM stopped offering the 4-cyl. option for ’12, choosing to go with the eAssist mild-hybrid powertrain for base models. A direct-injection 3.6L V-6, another former Ward’s 10 Best Engines winner, also remains available.

“Auto-stop was a pleasant surprise in a fullsize sedan,” says editor Byron Pope. Schweinsberg adds: “Powerful, quiet and smooth.”

But with fuel economy as the principal measuring stick for this contender, the LaCrosse eAssist fails to deliver and joins a long list of Ward’s 10 Best Engines hopefuls to wilt under the bright lights.

jamend@wardsauto.com