Out on Main Street America, it's back to school season for the kids. And on that note, it seems U.S. automakers have learned a lesson from their Japanese counterparts: You can't construct crummy cars and expect to purvey them for premium prices.

For evidence of Detroit's realization, turn with me in your textbooks to the August edition of the 2007 Vehicle Dependability Study (VDS), published annually by McGraw-Hill (NYSE:MHP) subsidiary J.D. Power and Associates. There, atop the rankings of the car makes with the fewest reported problems, we find none other than Detroit's own Buick, tied with Lexus for first place for the first time ever. (The rankings are based on problems per 100 vehicles, three years after production.)

The rest of the Big Three
Let's quickly run down how the major players fared in this latest edition of the VDS. First up, Buick parent General Motors (NYSE:GM) fared pretty well this year, with two of its brands -- Buick and Cadillac -- placing in the top five out of 38 auto makes surveyed. Its more pedestrian brands -- Chevrolet, Pontiac, and GMC -- didn't do quite so well. Both of the last two came in worse than the industry average of 216 problems reported per 100 vehicles. And Chevy, which lost its top-half status last year, remained in the bottom 50% in 2007. Back on the plus side, Hummer, which has always had a bad rep for quality, still does -- but it's improving rapidly.

Next up, Ford (NYSE:F). After two wonderful years enjoying a better-than-average reputation for quality, the flagship brand of Ford Motor Co. fell to a below-average 18th place in 2007. Mercury and Lincoln, however, remain in the top half of the rankings. In fact, Mercury made the top five for the second year in a row.

Rounding out the Big Three is Chrysler, which did its level best to prove DaimlerChrysler (NYSE:DAI) a genius for dumping this wreck earlier in the year. The VDS shows Mercedes-Benz climbing into lucky 13th place in the survey, above the industry average. Meanwhile, the jilted Chrysler division continues to plummet in the rankings. Sitting in the top half two years ago, Chrysler slipped to one slot below the average last year, and finds itself in 28th place in 2007.

The other Big Three
Meanwhile, in the land of the Rising Sun, things are as constant as, well, the sun rising in the east. Both Toyota (NYSE:TM) and Honda (NYSE:HMC) are sitting pretty with above-average ratings. It almost goes without saying that their luxury divisions are doing even better.

Meanwhile, Carlos Ghosn's problem child's problems continue. Nissan (NASDAQ:NSANY) is doing even worse than Chrysler -- ranked 30th out of 38 brands surveyed. And even Nissan's own luxury division, Infiniti, is in peril of losing its elite cachet. At 14th place, the brand is parked bumper-to-bumper with the industry average.

The point
I promised to put all of this into context for you, by drawing from this month's news an investing lesson. Here then, is that lesson. Quoting from the VDS: "Vehicle models that demonstrate strong dependability lose their value less rapidly compared with vehicles that are not as dependable. ... Automakers may reap numerous benefits from producing dependable vehicles ... higher residual values, decreased warranty costs [... and an] increased likelihood of customers recommending or purchasing newer dependable models." In investing terms, these laudable goals have the following effects:

Higher residual values
The faster a buyer thinks his new car will depreciate, the less he'll be willing to pay for it. It's only logical that if you know your $20,000 vehicle will, on average, resell for just $11,200 in three years, you'll want to cut your losses a bit. Demanding cash back, or perhaps 0% financing, are both tricks that GM has famously used in the past -- in full knowledge that bribing customers to buy its products in this way is mortgaging its future.

But what if GM can build on the success of its Buick brand? Should GM reclaim a reputation for quality, car buyers might become less fearful that a pricey new Buick will soon become a devalued used Buick. They might then be more willing to pay something more closely approximating the actual list price (gasp!). Result: Stronger gross margins -- and with GM currently eking out a living on a 4% operating margin, every extra basis point counts.

Decreased warranty costs
But building better cars doesn't just help an automaker's margins by inflating its top line. It can boost margins by cutting costs, as well. At the risk of stating the obvious, cars that break down during the warranty period cost money to fix. Money that, by definition, the maker is shelling out. Conversely, cars that don't break down don't incur this cost -- expanding margins from the other side of the equation.

Increased quality means increased sales?
Improving its quality, and its reputation for quality, can also help repair another peculiarly Detroit-ian bugaboo: loss of market share, with consequent declining sales. Maintaining scale is key to maximizing profits in a manufacturer such as GM, which explains why the company has been willing in the past to sacrifice margins in an effort to shore up sales. But if building better products will help GM attract more repeat buyers, and even expand its market share through customer recommendations, GM should be able to boost sales without sacrificing margins.

So any way you cut it, scoring high on the annual J.D. Power reports is key to Detroit's recovery, to GM's income statement, and to investors' returns on their GM stock. Or as GM might put it: "Honest, we fixed it. Are you sure you wouldn't really rather have a Buick?"

Discuss this article on the Fool's own Buying and Maintaining a Car board.

Fool contributor Rich Smith does not own shares of any company named above. Nissan is a Motley Fool Global Gains selection. The Motley Fool's disclosure policy requires him to tell you that, but it doesn't say he has to tell you he drives a battered Chevy S-10. (Oops.)