May sales numbers show a robust month of new car sales. While that's good news for car companies, new car transaction prices were also high, which means consumers are paying more for new cars.

The 2014 Cadillac ATS.  Cadillac posted
The 2014 Cadillac ATS. Cadillac posted the largest sales gain in the industry for May. (Photo courtesy Cadillac)

According to data from Automotive News (subscription required), total automotive sales for May increased 8 percent compared with May 2012. So far in 2013, car sales are up 7 percent compared with the same period last year.

Cadillac and Porsche saw the biggest sales gains, with 40 and 38 percent increases compared with May of 2012, respectively. Cadillac says the sales gain are the brand's biggest increase since 1976 and makes the Cadillac the fastest growing automotive brand in the country. Infiniti, Mitsubishi and Mini posted the biggest sales decreases of mainstream brands. Infiniti sales were down 25 percent compared with May 2012. Mitsubishi sales were down 15 percent and Mini sales were down 3 percent.

Of American automakers, Ford (including Lincoln) had a 14 percent sales increase compared with May of 2012. Chrysler, including its Jeep, Dodge, and Ram brands posted an 11 percent sales increase. General Motors had a 3 percent increase compared with May 2012.

In a press release, Kelley Blue Book says that May new-car transaction prices rose along with car sales. Compared with April, KBB says that transaction prices are up 0.5 percent. That works out to consumers paying an average of $152 more on each new car.

As the increase in transaction prices show, strong car sales aren't always great news for consumers.  As demand increases, car companies don't need to discount their products. If you want the best deal on a new car, it may pay to wait until car sales slow down. If you can't wait, brands with weak sales are likely to have the best discounts. For example, Honda Motor Company sales, which include the Honda and Acura brands, only saw a five percent increase in sales compared with May 2012. KBB reports that their transaction prices in May were actually 0.4 percent lower than they were in April as dealers dropped prices to try to spur sales.

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