Hot on the heels of an estimated 20,300-unit September production shortfall, North American car and truck assembly plants are embarking on an even more robust fourth-quarter program than the one put in place a month ago.

Although Ford has not yet revealed what revisions it has in store for October-December, the rest of the industry already has boosted production plans by some 53,900 units for the period.

The bulk of the increase is set to occur this month with 61,700 units added to the October slate, some of which likely were deferred from September and some pulled forward from December’s plan, now 16,400 units lower than it was a month ago.

But, even assuming the entire September and December downward volume adjustments are included in this month’s higher schedule, that still means an additional 16,700 vehicles in the October boost, on top of which some 8,600 units have been added to the November plan.

The largest Q4 increase comes from General Motors, up 18,500 units, mostly cars, including gains of 26,000 in October and 3,000 in November, followed by a 10,500-unit cut in its December slate.

Chrysler has boosted its October-December plan by 8,500 units, including an October increase of 9,000 that is partially offset by a 500-unit cut in December. Unlike GM, where most of the gain is in cars, Chrysler tilts toward trucks, up by 5,000 for the quarter.

Other quarterly gains include a 22,000-car boost at Volkswagen de Mexico, 12,000 in October alone; 4,800 at Toyota (4,000 in October); and 8,000 at Nissan spread across all three months, with trucks accounting for 4,500.

Incorporating the revised outlook, North American plants now are in line to build some 15,692,200 cars and trucks this year, up 16.5% from 2011’s 13,471,500 completions.

That includes a 7.8% increase for the Detroit Three that will end the year with a 52.9% output share vs. 57.2% in 2011, when Japanese transplant operations were hobbled by natural disasters abroad.

With Japanese North American plants back in full swing this year, transplants are set to build 29.2% more vehicles than they did in 2011, accounting for a 44.7% output share compared with 40.3%.

Dedicated medium- and heavy-duty truck makers, after trimming some 3,800 units from their Q4 plans, still are slated to build 9.3% more vehicles this year, accounting for 2.4% of industry production compared with 2.5% in 2011.

abinder@wardsauto.com