Combined sales of full-size SUVs in the US dropped 25% in May, compared to the same period last year.

According to company reports, combined sales of full-size SUVs (models longer than 193 inches) dropped to 135,137 units during May 2005, down from 180,808 the year before. Excluded from this are the sales for Isuzu’s Ascender, which were not available. Given that Isuzu is in steep decline, the sales of the Ascender represent a rounding error in this calculation.

(All percentage calculations here are based on actual volume, not the DSR which represents an averaged daily sales rate based on the number of selling days in the month. For a more complete and entertaining explanation on DSRs and sales numbers, see Car Buyer’s Notebook here.)

For the first five months of the year, sales of full-size SUVs have dropped 20% compared to 2004. The only automaker who saw gains in its full-size lines was Nissan, with the Armada and the Infiniti QX56. Unlike last month (earlier post), even the Dodge Durango succumbed to the market shift.

Again, Ford and GM are suffering especially. That financial pain comes despite some solid successes for both automakers, such as the Ford Mustang, which rose 47% on the month to a robust 19,721—now outselling the Chrysler 300. GM’s Cadillac brand had its best May month since 1993, and a number of new models (e.g, Cobalt, Equinox, Aveo) are selling very strongly. That doesn’t offset the huge void left by the cratering of the full-size SUV sales—hence the credit downgrades that have been in the news.

“Ever seen erosion when water runs down the hill and you can’t stop it? It’s going to be more of the same with the domestics continuing to lose ground and the Asians continuing to gain ground,” said Frank Ursomarso, a Wilmington, Delaware, dealer. He sells GM’s Pontiac and GMC brands, Ford’s imported Volvo and Jaguar cars, and BMWs from Munich, Germany-based Bayerische Motoren Werke AG. (Bloomberg)

How much of this shift is due to gas prices and uncertainty? Some, but the rejection of the full-size SUV doesn’t represent a wholesale shift in the consumer zeitgeist toward fuel efficiency. Crossovers, minivans and small SUVs are doing well. Chrysler continues to post strong sales for its large cars and vans. The all-new HEMI-powered Dodge Charger racked up almost 2,000 units in sales in just 10 days.

But the trend award from the largest is undeniable.