Automakers and dealers enter the waning weeks of the model year with a glut of slow-selling cars.
Meanwhile, General Motors has been unable to come out of a deliberate overstocking earlier this year.
Normally July and August tie for the leanest stock levels of the year, both averaging a 56-day supply. But the industry entered this August at 69 days, tying the Aug. 1, 2008, record. Measured in units, the Aug. 1 industry inventory of 3.9 million unsold light vehicles rose 324,600, or 9.1 percent, from last August, also the highest for this date.
Again, GM provided almost 80 percent of the year-over-year growth. The automaker's Aug. 1 inventory jumped by 256,400 units, or 38 percent, from a year ago.
Earlier this year, GM said it overbuilt some models because it planned plant downtime for retooling in the second half. U.S. sales boss Kurt McNeil said GM would "strategically" cut production rather than boost incentives or "dump vehicles into daily rental fleets."
In July, GM cut spiffs 5.7 percent, Autodata said. And it slashed rental fleet sales. July sales fell 15 percent, so its supplies remain high, declining one day to 104 days.
Excluding GM, the industry added 68,200 units, or 2.4 percent, during July to 2,958,200.
Fiat Chrysler and Nissan North America started the month with larger supplies of trucks than cars, and Mercedes had an equal supply, but all other automakers had greater car stocks.
American Honda had a 61-day supply of trucks but 40 days of cars. Volkswagen Group had 120 days' supply of cars, but trucks at 43.