This week I’m driving a Buick Verano. Last week I had a BMW 528i xDrive. The first, based on the Chevrolet Cruze, is the smallest car Buick has offered since the J-Body Skyhawk was discontinued back in 1989. It weighs in at 3,300 pounds (midsize territory) and is powered by a 180-horsepower, 171 pound-feet 2.4-liter four-cylinder engine. The latter, for those who don’t speak recent BMW, is the marque’s midsize sedan with a 240-horsepower, 260 pound-feet 2.0-liter turbocharged four-cylinder engine (yes, a four in a $60,000 car) and all-wheel-drive. So configured, the neu 5er checks in at 4,000 pounds. Which would you expect to get better gas mileage?

I’ll spare you the suspense: despite 60 more horsepower, all-wheel-drive, and 700 additional pounds, the BMW bests the Buick. The EPA ratings suggest the slimmest of victories, 22 city and 32 highway for the BMW, 21 and 32 for the Buick. In my suburban driving, though, the trip computer in the BMW often reported about 25 mpg, while that in the Buick tended to be in the low 20s. With a very light foot you’ll manage high 20s in either car. A marginal result for the Buick, and a surprisingly good one for the BMW.

How did BMW manage this? To begin with the new 2.0-liter turbocharged four-cylinder engine seems to be very efficient, while also accelerating the heavy sedan energetically when called upon to do so (a heavy foot will sink gas mileage into the high teens). It’s paired with an eight-speed automatic, vs. the much more typical six-speed in the Buick, and often automatically shuts off when the car is waiting at a light. Finally, recent BMWs have an “Eco Pro” mode which is essentially the opposite of the “Sport” mode. Select it, and throttle responses become quite Prius-like. So hobbled, the car doesn’t so much seem sluggish as pleasantly relaxed. Well, at least not from the driver’s seat. Moving languidly away from one light I had an actual Prius tailgate then pass me!

What does this mean? In this and other cases, it’s clear that after years of stagnation the average car’s gas mileage is increasing. If a heavy all-wheel-drive BMW can manage mid-to-high 20s in suburban driving, then a mainstream midsize sedan should be capable of mid-30s. And the Buick, which is a very nice car in other ways? It’s engine has some catching up to do.