Low inventory, especially among Japan auto makers, will plunge in May’s seasonally adjusted annual sales rate to a 6-month low, according to a Ward’s forecast.

The report projects U.S. light-vehicle sales of 1.12 million units over 24 selling days, a 10.2% increase in daily sales, compared with year-ago.

At forecast levels, LV sales equate to a 12.4 million-unit SAAR, the lowest since November and 600,000 less than the year-to-date through April.

The expected decline primarily is due to shrinking inventories among Japanese auto makers, whose domestic and North American production continues to be disrupted as a result of the Japanese earthquake and tsunami in March.

Related document: Ward’s U.S. Lt. Vehicle Sales and Inventory Forecast

Lower inventories in general have reduced incentive spending across the U.S. industry, and several auto makers have instituted price hikes in the last 60 days, likely pushing some potential customers to the sidelines for the time being.

Nonetheless, the industry’s DSR is expected to track more than 9% above April, as U.S. and Korean auto makers push to meet pent-up retail demand.

The Ward’s forecast shows the Detroit Three claiming nearly 48% of the market, their highest combined share since December 2008.

General Motors is expected to deliver more than 230,000 vehicles, a 12% year-over-year increase in DSR, good for a 20.6% market share. Ford is expected to take a 16.7% share and Chrysler will account for 10.3% of LV deliveries.

Hyundai and Kia, which combined for a record 9.4% U.S. share in April, likely will break that record with a minimum 9.8% in May. The two auto makers could increase their combined share, though they are hampered by limited domestic production capacity.

Toyota’s DSR is forecast some 14% below year-ago, dropping the auto maker’s share below 12.5% for the first time since July 2005.

Honda’s daily sales are forecast to trail year-ago by 5.3%, equating to a 9.9% share, compared with 10.8% in April. Nissan is expected to grab 7.2% of the market as it keeps some incentives in place.

The growth rate for the small-car segment, bolstered by improved product lines and rising fuel costs over the first four months, should taper off in May as a result of supply issues.

But demand for fuel-efficient vehicles continues to grow and likely will result in another share spike for the segment this summer as production ramps up again.

With news that supply issues should be resolved sooner than was generally thought a month ago, Ward’s continues to forecast 13.2 million LV deliveries for 2011, as May and June sub-13 million-unit SAARs are offset by above-trend volumes in the third and fourth quarter.

Projected sales bring May’s year-to-date deliveries to 5.32 million units, a 15.2% gain on like-2010.

jsousanis@wardsauto.com