CNBC adjusts sales percentages for the number of selling days in the month. There were 26 sales days this April, versus 24 last April.
Though GM's car sales declined by only 7.8 percent, it sold 32.3 percent fewer trucks in April than it did a year ago.
GM's Hummer brand was hit especially hard, with a 49.8 percent plunge year-over-year. The company's GMC division reported a 29.6 percent fall.
GM's inventory fell to about 824,000 vehicles at the end of April, down about 206,000 vehicles from a year earlier, to the lowest level since September 2005.
GM said a two-month-old strike at supplier American Axle & Manufacturingreduced the automaker's April production by about 130,000 vehicles.
Ford Motor's U.S. vehicle sales for April dropped 19 percent on an adjusted basis, including a 24.6 percent decline in truck sales.
Ford said overall sales declined to 200,727 vehicles from 228,623 a year earlier, including sales for its Volvo, Jaguar and Land Rover brands.
Ford sold a total of 120,814 trucks in April, down from 147,891 last year.
Retail gas prices began the month at $3.26 a gallon nationwide last month but had crept up to $3.57 per gallon by the last week in April, according to the U.S. Department of Energy. They set a record on April 30, rising to a national average of nearly $3.62 a gallon.
Pickup sales have been falling for months because of the slowdown in housing construction, and the trend away from SUVs began several years ago as Baby Boomers aged and roomy but more fuel efficient crossover vehicles gave consumers more choice. But automakers said gas prices are accelerating the trend.
"At $3 a gallon, there's a lot of discussion about the price of gasoline but not much change in behavior. But in the mid-$3.50 range with $4 on the horizon, there's a lot of change in behavior," said Mike Jackson, the chief executive of AutoNation, the country's largest auto retailer.
Jackson said it will be interesting to see if the change is permanent or if consumers return to larger vehicles once their initial shock at the pump has worn off and the economy improves.
Toyota Sales Down, Led Lower by Light Trucks
Toyota Motor experienced a smaller decline than its U.S. rivals, but one that was also led lower by diminished truck purchases.
Toyota reported a 4.5 percent fall in April U.S. auto sales to 217,700 vehicles after adjusting for the two extra selling days in the month, with sharp drops in SUVs and pickup trucks, like the FJ Cruiser and the Tundra, more than offsetting gains for small cars like the Yaris and the Prius hybrid.
The automaker's light truck sales fell 14.9 percent. Passenger car sales actually rose, by 3.3 percent on an adjusted basis.
Toyota said its Toyota-brand sales were down 2.7 percent, while sales for its luxury Lexus brand fell 17.1 percent.
In the United States—the world's largest auto market—April marked the fifth consecutive month of sales declines for Toyota, now the world's top automaker by global sales volume.
- Wire services contributed to this report.