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These cars will cost you, if you can find one

May was a good month for BMW.
Image: Audi S4
An Audi S4 will set you back around $46,000. If you can actually find one.Audi
/ Source: Forbes

May was a good month for BMW. The German automaker reported 22,092 units sold last month, with total sales up 5 percent over the same time last year. It was BMW's best result so far this year, despite a year-to-date decline of almost 4 percent. Executives are optimistic about summer sales — particularly the prospects for the new 5 Series sedans, which go on sale later this month.

"Sales are where we expected them to be," says Thomas Salkowsky, a spokesman for BMW North America. "We expect the new 5-Series to have a strong Q3 and Q4."

But it may be difficult to get your hands on one of these $49,600 sedans. BMW's 5-Series vehicles rank with the Subaru Legacy and Volkswagen GTI among the hardest-to-get cars in the U.S.

To compile our list of hard-to-get cars, we used inventory data from Wards Auto, an automotive data and analysis firm based near Detroit. We calculated average day-supply rates of 2010 model-year vehicles for March, April and May to figure out, on average, just how quickly these cars leave the lot. (Day-supply rates are inventory numbers, divided by the daily selling rate that month.) Then we spoke with manufacturers to get a street-level sense of how long cars typically wait for buyers, since day supply data can fluctuate from month to month.

In compiling this list, we did not include exotic and super-luxury cars (think Lamborghini and Rolls-Royce), special-edition models, or models from discontinued brands like Saturn, which can be hard to find for reasons other than high demand. Instead, we exclusively considered cars (not trucks or SUVs) which are affected by other sales factors, and typically are produced in smaller numbers, with shorter day-supply rates.

Imported vehicles dominate the list. Several specialty cars made the cut — the $114,200 Audi R8 has racetrack speed (0-60 mph in 4.2 seconds, with a 190-mph top seed), and the domestic $25,140 Dodge Charger has a retro muscle-car cult following — but it's the mainstream stuff that gets the most attention.

Audi manufactures several vehicles on our list: The S4, S6 and R8 each qualified as a fast-turner. But the $36,000 A5 stood out the most last month, when it broke its own sales records. In May the A5/S5 model line (which includes A5/S5 cabrio sales) sold 1,757 cars, a year-over-year increase of 145 percent.

Christian Bokich, a spokesman for Audi, says company averages a 30-day supply for cars and trucks across its range. "Typical dealer stock among luxury and non-luxury brands is 60-90 days, so you can see how tight we've kept inventory," he says.

But Bokich also says that since vehicle availability varies by region and city, customers should stay in contact with their local Audi dealer to discuss availability options. Any particular car could be in one of several places: on-site, located at another dealership, or en route to the showroom from the factory.

One reason why imports rank high on our list is the different business strategies used by domestic and foreign brands. One official at a German automaker said he keeps supply lines tight so that dealers don't discount the cars, thereby upholding residual values, and promoting brand and luxury value to the dealer and to customers. (It basically ensures that his cars are in "only the right driveways," he said.)

Imports are also hard to get because of limited supply lines, and how many production lines are open for a given model. Sales of Audi's popular Q5 SUV were limited earlier this year because the company had yet to open a second production line in China, forcing it to meet worldwide demand just off its Germany plant.

Foreign automakers can also hold new vehicles on the dock and not put them up for sale, in an effort to control the time when their product becomes counted as inventory. "When a car is built here, it enters inventory more or less directly," says John Sousanis, an automotive industry analyst at Wards. "If a car is built somewhere else, they don't have to ship it here, necessarily. They could take orders on it. Importers just have more options about what they do with those vehicles when they import."

Of course, many foreign brands manufacture hundreds of thousands of cars in the U.S. as well, including BMW in Spartanburg, S.C.; Volkswagen in Chattanooga, Tenn.; and Mercedes-Benz in Tuscaloosa, Ala..

Still, if you're considering something from Audi or BMW, better start visiting dealers now. Just to be on the safe side.