According to the Detroit Free Press, U.S. automakers are sitting on an overflow of unsold cars and trucks that is the largest backlog in U.S. auto history.

The news source says stockpiles, backing up on auto-dealer lots, at assembly plants and wherever else automakers can put them, could help consumers as automakers use pricier incentives to spur demand. Otherwise, a larger production cut at assembly plants could be in the offing to trim supply.

The paper says about 3.93 million unsold vehicles are sitting on dealer lots, in transit from the plants where they were made or at hushed-up overflow sites like the Michigan State Fairgrounds. All told, about 630,000 more unsold cars and trucks are sitting around than a year ago. Since March, inventories have been at all-time highs.

According to the Detroit Free Press, there are several reasons for the jump. One is poorer-than-expected sales this year. Another is that Ford Motor Co. is building extra F150 pickups because its Norfolk, Va., and Kansas City, Mo., plants will be down for several weeks through July while launching the new F-series.

A third reason, but one the automakers won't concede: the UAW negotiations this summer. A backlog of unsold cars and trucks gives the automakers a buffer in case there's a labor strike, while also acting as a subtle, unspoken bargaining chip.