GM is having trouble moving its full-size trucks.

General Motors says its 8 percent drop off in pickup truck sales during the month of November was largely due to heavier incentives offered by cross-town rivals Ford and Chrysler. GM's overall sales ticked up 3 percent during the month.

GM says its incentives for its full-size trucks, the Chevrolet Silverado and GMC Sierra, were about $500 less than the industry average, while the segment's "biggest spenders" were putting between $1,500 to $1,700 more than GM on the hoods of their trucks.

GM claims Chrysler was the biggest spender during the period, giving buyers "five thousand bucks a unit" off the list price of its Ram pickup.

"We don't think that's sustainable," GM U.S. sales chief Alan Batey commented to Automotive News, referring to Chrysler's deep discounts.

However, the playing field wasn't exactly level in November. In anticipation of an all-new truck arriving for the 2014 model year, GM launched its 2013 trucks about 45 days before it typically would. That means GM dealers were pushing higher priced 2013 model year trucks in November while both Ford and Chrysler were trying to clear out their remaining 2012 models.

GM estimates that about 60 percent of its truck sales last month were 2013 models, compared to 35 percent for Ford and less than 10 percent for Chrysler.

However, GM could be forced to offer more cash back as its pickup truck inventory has swollen to 245,853 units, or a supply of 139 days. In comparison, GM had a 110 day supply on hand at the end of October. GM considers a pickup truck inventory between 200,000 and 220,000 units ideal.