DETROIT — The popular Cash for Clunkers program boosted Ford Motor's U.S. sales in August by 17.2 percent over last year, but shortages of smaller vehicles weighed on rival Chrysler.

The program, which ended on Aug. 24, drew hordes of buyers into quiet showrooms by offering up to $4,500 toward new, more fuel-efficient cars and trucks. The hefty rebates gave automakers and dealers a much-needed lift, spurring 690,114 new sales, many of them during August, at a taxpayer cost of $2.88 billion.

Other automakers are expected to release U.S. sales figures later today. Combined, the results are likely to mark the first year-over-year monthly sales gain since October 2007.

Ford sold 181,826 cars and light trucks compared with 115,117 in August 2008, when high gas prices and growing economic uncertainty kept people away from showrooms.

Two of Ford's vehicles — the Focus and Escape — were among the top selling cars under the clunkers program. Sales of the Focus rose 56 percent while those of the Escape crossover vehicle climbed 49 percent.

Meanwhile, low supplies of fuel-efficient vehicles at Chrysler kept the automaker from benefiting more from the clunkers program, whose rebates encouraged customers to buy gas sippers in exchange for guzzlers with gas mileage of 18 mpg or less.

Chrysler sales fell 15 percent to 93,222 units.