The industry days supply dropped again in March, the second consecutive monthly decline.
There was a 55-day supply of light vehicles on April 1, down from 59 days a month earlier.
A year ago, inventories were higher by more than 500,000 cars and light trucks.
"Our problem is inventory - I need trucks," said Bill Lovejoy, General Motors group vice president of North American vehicle sales, service and marketing.
GM's days supply climbed slightly in March to 62 days, up from 58, including small increases in both cars and light trucks.
GM's sales in March were 4.9 percent below the year-ago month, but that included a 4.7 percent increase in light-truck sales.
Besides trucks, Lovejoy complained that GM could sell more cars such as the Chevrolet Impala if there were more inventory. The Impala had a 35-day supply April 1, up from 26 a month ago.
On April 2, GM raised its second-quarter production forecast for North America to 1.48 million, a 4 percent increase from the last forecast.
Ford Motor Co. also is likely to increase its second-quarter production schedule Wednesday, April 17, when the company reports its first-quarter financial results, said George Pipas, sales analysis and reporting manager.
In March, Ford whittled its days supply to 64, from 72 a month earlier.
DaimlerChrysler cut its U.S. inventory to 56 days, from 64.
As usual, import brands had much lower inventories than the Big 3.