????Infiniti finished 2011 with a flurry of sales, upping its December results 37% from a year ago -- three times as much as the industry's overall 11% climb.

????That capped an impressive year in which Nissan's upscale brand moved 103,411 cars and trucks, a 28% increase. One German luxury car manufacturer reports that while it has been stealing customers from its other European competitors, it has been losing some buyers to Infiniti.

????Infiniti's recent success is due to its developing identity as a sporty upscale brand that combines performance and innovative styling with Infiniti's traditional quality.

????That image is epitomized by the G series -- which accounts for more than half of Infiniti's sales -- and its flagship car, the G37 sedan.

????Since the G37's arrival on the market two years ago, Infiniti has been trying to extend the G series down-market with a smaller-engine version called the G25.

????It's a sales tactic perfected over the years by German manufacturers. Launch the flagship first and then introduce lesser variants with different engine sizes later on. Thus a Mercedes S550, launched with a 5.5 liter engine might be complimented a few years later by an S430 at a lower price point. The aim is to give customers a new reason to shop the brand.

????The G25 ought to do that. To outward appearances, it is identical to the G37. The differences are under the hood -- and on the window sticker.

????The G37 with all wheel drive is powered by a 3.7 liter V6 putting out 328 horsepower. Its price starts at $36,650.

????By comparison, the G25 makes do with a 2.5 liter V6 putting out 218 horsepower. Base price is $33,950 -- $2,700 less.

????In order to further hold down the price, Infiniti doesn't offer a six-speed manual transmission or navigation system for the G25.

????So for a little less money, you get a little less car. Sound like a reasonable tradeoff?

????On the plus side, the smaller engine gives the G25 a weight saving of 62 pounds versus the G37. The lower weight and smaller engine pay off in fuel economy. The G25 is rated at 19 mpg city/27 highway versus 18/25 for the G37.

????That's all good, but it has to be weighed against the deterioration of performance in the G25.

????While the G37's admirable dynamic characteristics remain intact with the smaller engine, the G25 seemed surprisingly sluggish by comparison. A sport sedan should respond with enthusiasm when you depress the accelerator; the G25 seemed uncomfortable doing so. It really misses the extra oomph of the G37's larger engine.

????Other reviewers observe no such shortcomings, but I couldn't get over the feeling that I was driving a pale imitation of the real thing in the G25.

????The problem may not lie with the car but with the abbreviated nature of the Infiniti product line. Besides the G series, it consists of only one other sedan, the more expensive M series.

????With cars in only two sizes, Infiniti is going up against more amply supplied rivals like Audi, which sells slightly fewer units but offers many more models, including the A4, A6, and A8.

????So Infiniti's two lines have to cover more of the market. The midsize G25 becomes the brand's entry point, rather than a smaller car in the same class as the A4.

????Infiniti may also want to investigate the practices of the Detroit Three, who typically initiate a product line with a lower-priced version and then introduce higher-horsepower variants later on.

????Cadillac, for example, waited two years before showing off the V high-performance addition to the CTS line.

????The G25 might have been more appealing had it appeared ahead of the G37 instead of behind it.