When it comes to the Buick LaCrosse’s powertrain, the theme is gradual evolution rather than revolution, and that may suit “if it ain’t broke don’t fix it” shoppers perfectly.

Propelled by one of two six-cylinder engines, the 2005 Buick LaCrosse runs just fine on good ol’ 87-octane fuel. CX and CXL models house a 3.8-liter, single overhead cam, 12-valve, cast iron V6 good for 200 horsepower at 5,200 rpm and 230 lb.-ft. of torque at 4,000 rpm. Fuel mileage is rated at 20 mpg in the city and 29 mpg on the highway. The 2005 Buick LaCrosse CXS is motivated by a 3.6-liter, dual overhead cam, 24-valve, aluminum V6 that ups the horsepower to 240 at 6,000 rpm. Torque is rated at 225 lb.-ft., but peaks at a very low 2,000 rpm. This more powerful V6 includes variable valve timing, and fuel mileage dips a bit to 19 mpg in the city and 28 mpg on the highway.

Under most driving conditions, the 240 horses in our CXS tester did a fine job of moving the LaCrosse through city and highway traffic, there was plenty of torque for quick takeoffs, and even at full revs the engine was composed and quiet. But don’t confuse quick with fast: Expecting driving excitement from this Buick is a setup for disappointment. The 3.8-liter V6, with 40 fewer ponies, exacerbates the power deficiency.

Regardless of which engine is under the hood, the only transmission choice is a rather old tech four-speed automatic, lacking the adaptive and manually-interactive shifting features found on many more contemporary gearboxes. This tranny is a tried-and-true part of GM’s powertrain arsenal, and manages the engines’ power with smooth shifts and relatively little gear hunting. But, after a week of mixed driving, our 2005 Buick LaCrosse CXS tester returned a rating of only18.5 mpg – a figure that would probably be bettered with a more efficient five- or even six-speed automatic transmission.