U.S. auto sales continued at a brisk pace in April although the total volume of vehicles sold increased only slightly due to a quirk in the calendar, lower sales to rental fleets and weaker deliveries from General Motors Co. and Ford Motor Co.

Annualized sales were at a 14.4 million pace, according to researcher Autodata Corp., indicating that auto purchases in the U.S. remain on track to rise by 10 percent or more this year compared with 2011. Auto makers expressed confidence in the sales over the remained of the year. GM on Tuesday said it has boosted its industry sales forecast to between 14 million and 14.5 million new cars and trucks compared with a previous outlook of between 13.5 million and 14 million vehicles, according to The Wall Street Journal.

"We expect gradual improvement in the economy going forward," Don Johnson, GM's vice president of U.S. sales operations, said in a conference call on Tuesday. That trend, he said, would allow consumers to "gain even more confidence and drive vehicle sales higher for both the industry and GM."

Overall U.S. new-auto sales for the month rose 2.3 percent from a year earlier, to 1.18 million cars and light trucks, Autodata said.

April was a mix with Chrysler Group LLC and Toyota Motor Corp. reporting strong gains while others posted declines. Part of the reason was that last month had three fewer selling days than April 2011, an unusual occurrence that can lower sales totals by as much as 11 percent, according to analysts.

GM said it was hurt by a big drop in sales to fleet operators, such as car rental companies. Its sales to consumers were flat. Overall, GM's April sales fell 8.2 percent to 213,387 cars and trucks. Ford had a big month of fleet sales, which contributed 37 percent of its total, but its April light-vehicle sales fell 5.1 percent, to 179,658, amid drops in sales of the Escape sport-utility vehicle, Fiesta compact and Crown Victoria sedan, which has gone out of production.

"The momentum we have seen at the start of the year is continuing and there is no indication of a slowdown," said Jesse Toprak, an industry analyst with automotive website TrueCar.com. "The industry kept the same selling rate and best yet, it lowered incentives."

Chrysler, meanwhile, reported a 20 percent increase to 141,165 vehicles, including Fiat autos. Sales of its passenger cars, which were recently revamped, climbed 37 percent. Chrysler is majority owned by Italy's Fiat SpA.

One bright spot for all three Detroit auto makers was higher sales of pickup trucks, which bring in big profits. Sales of GM's GMC Sierra and Chrysler's Ram trucks both rose about 19 percent. Chevrolet pickup sales rose 5 percent.

Ford, whose pickup sales increased 4.4 percent, got help from buyers like Joe Opsahl, whose family construction company, RMK Concrete Foundations, is based in Stockbridge, Mich. A few weeks ago, he had enough of his 2002 Mercury Mountaineer and decided to do something he hadn't done in 20 years—he bought a brand new vehicle.

"I've always bought used, and it was about time I got something new," Mr. Opsahl, 38 years old, said from a job site. Using a discount available to his wife, he paid $27,000 for a silver Ford F-150 truck with a six-cylinder engine. "The money was there, and a lot of work is coming in, so I was comfortable" buying a new truck.

Toyota, bouncing back from production disruptions of a year earlier, when Japan was hit by a massive earthquake, said its U.S. sales rose 11.6 percent in April from a year earlier to 178,044 vehicles.

"There is no doubt the Japanese auto makers will be back in the market with high inventory and competitive in fleet sales," GM's Mr. Johnson said.

But Honda Motor Co. and Nissan Motor Co. slumped compared with a year ago. Honda's sales fell 2.2 percent to 122,012 vehicles, despite heavy discounts and promotions on the Civic compact and other top-selling models. Nissan's sales declined less than 1 percent to 71,329 vehicles, with Nissan brand sales off 0.9 percent and Infiniti brand sales up 5.4 percent.

Sales of GM's battery-powered Chevrolet Volt fell 36 percent from March to 1,462 vehicles. GM halted production of the Volt in mid-March due to slow sales and resumed production a week earlier than planned last month. The results were still more than the 493 the company sold in April 2011.

Volkswagen of America said it sold 37,525 vehicles in April, a 31.5 percent increase over a year earlier. Its sales of the Jetta sedan, the auto maker's best seller, were about flat for the month compared with April of last year. Company officials said they haven't seen evidence that the newly redesigned Passat was taking sales away from the Jetta.

Volkswagen is expanding its U.S. operations, saying last month that it will add 150 new jobs through 2018 as it builds a 30,000-square-foot expansion to its credit office in Libertyville, Ill., scheduled to be completed early next year. Earlier the company said it would add 1,000 jobs at its manufacturing plant in Chattanooga, Tenn., to meet growing demand for the 2012 Passat.

Luxury-car makers again delivered big gains. U.S. sales of Daimler AG's Mercedes brand vehicles rose 23.8 percent compared with a year earlier to 22,336; BMW AG said its BMW brand rose 12 percent to 21,062; Audi brand sales rose 15 percent to 11,521 vehicles.