Mexico's new-vehicle sales tumbled 37.1 percent in May from May 2008, to 53,683 vehicles.
New-vehicle sales in Mexico have fallen every month, on a year-to-year basis, since August 2008.
For the first five months of this year, new-vehicle sales in Mexico dropped 30.5 percent, to 300,564.
General Motors hung on to its No. 1 ranking among automakers in May even though its sales slid 46.3 percent to 10,194.
That left GM just 554 units ahead of No. 2 Volkswagen AG, whose sales slipped 13.3 percent to 9,640.
VW edged past Nissan Motor Co., which had been No. 2 a year earlier. Nissan's sales fell 45.7 percent to 9,434.
Ford Motor Co. moved up a notch from its year-earlier ranking to No. 4, as its sales dropped 31.5 percent to 6,854.
Sales at No. 5 Chrysler LLC tumbled 43.5 percent to 5,682.
Car sales fell 38.7 percent to 30,476 in May. Truck sales fell 35.0 percent to 23,207.