NEW YORK (CNN/Money) - Automakers posted strong gains in U.S. sales in August, as new incentives helped bring customers into showrooms.

Overall U.S. vehicles sales reached 1.7 million in the month, up 13 percent from the same period a year earlier, according to sales tracker Autodata Corp. Light trucks sales, which including pickups, sport/utility vehicles and minivans, gained 16.8 percent, and car sales managed a healthy 9.6 percent gain as well. The industry now appears on target to top 17 million vehicles sold for the fourth straight year.

The results were good enough for General Motors Corp. (GM: up $0.20 to $45.75, Research, Estimates) to raise its earnings guidance for the third quarter and full year.

The world's largest automaker said it now expects to earn 90 cents a share in the quarter, excluding special items but including the results from Hughes Electronics unit it is in the process of selling. That's up from its earlier guidance of 80 cents a share, and the current consensus analyst earnings-per-share forecast of 88 cents according to earnings tracker First Call. The company earned 85 cents a share on that basis a year earlier.

For the year, the company expects to earn $6.10 per share, up from its earlier guidance of $5.60 a share and First Call's forecast of $5.99. The company earned $3.23 a share in 2001.

As for August sales, GM posted an 18 percent gain in daily U.S. sales rate as it sold 489,471 new cars and light trucks in the period. GM calculates that August 2002 had one more sales day than the year-ago month, so the overall gain in sales increased 22.4 percent. Light trucks saw a 27.4 percent gain in daily sales rate, while car models posted a more modest 7.4 percent gain.

GM said that its year-to-date U.S. light vehicle sales are up 4.4 percent, as it has overcome weak demand for fleet purchases from businesses, especially car rental companies. The retail sales to consumers gained even more than overall sales, up 8 percent.

Ford (F: up $0.11 to $11.05, Research, Estimates), the world's No. 2 automaker, posted a 12.1 percent gain to sell 365,406 cars and light trucks, a record for August and the best sales month so far this year. The company sold 325,943 U.S. vehicles in August 2001. The company saw a 15 percent gain in car models and a 10.3 percent rise in the sale of light trucks. All of Ford's brands except Volvo posted a sales gain in the month.

"We're not where we want to be or where we need to be but we are improving," said George Pipas, Ford's head of sales reporting and analysis.

The company also announced it expected to build 960,000 vehicles in North America during the fourth quarter, up from the 940,000 it expects to build in the current period.

Chrysler, the North American unit of DaimlerChrysler AG (DCX: up $1.65 to $41.52, Research, Estimates), sold 210,855 vehicles, up from 164,555 in the same month a year earlier, giving it a 28 percent gain in overall sales, and a 24 percent gain in its daily sales rate due to an extra sales date in August this year by Chrysler's calculation.

Chrysler also saw gains across its various models and brands, as cars posted a 25 percent gain in daily sales and trucks posted a 23 percent gain.

Importers also did well. Japanese automakersToyota Motor Corp. (TM: up $0.35 to $48.30, Research, Estimates) and Honda Motor Co. (HMC: down $0.46 to $20.14, Research, Estimates) and Korean automaker Hyundai reported their best U.S. sales months in history in August, and several other overseas automakers, including German automaker Audi and Japanese carmaker Mitsubishi, reported their best U.S. August sales on record.

But for the most part the overseas manufacturers posted smaller percentage sales gains than recorded at GM and Chrysler, as those U.S.-based automakers gained making market share, while Ford slipped only slightly.

GM, Ford and Chrysler all offered buyers zero-interest financing to buyers of 2002 models in August, as they tried to clear dealer lots to make room for 2003 models. Chrysler also publicized an extended warranty on all new models during the month. GM extended the zero-interest offer to 2003 models Wednesday, while Ford expanded the offer as it applied to 2002 models.

The strong response has left GM with its lowest inventory levels since October 1994, when the company was dealing with the effect of both local labor strikes and some significant model change overs. Those inventories could constrain sales going forward. But Paul Ballew, executive director for market and industry analysis at GM, said that the company is pleased enough with demand and customer response to zero interest financing to extend the offer to the 2003 models on Wednesday.

"[The] first question everyone has is why (extend the incentive)," he said to analysts Wednesday. "We plan to stay aggressive. We're playing to win, not playing to draw."

Despite the strong month, Ford's and Chrysler's year-to-date sales show declines in the face of GM gains. Ford's overall sales are off 6.5 percent, while Chrysler Group sales are off 1 percent.