 •   Aftermarket parts volume benefited from stronger retail markets, reflecting improving freight volumes and aging  truck fleets. This higher market demand resulted in increased aftermarket parts sales volume of $258.8 million  and related cost of sales of $157.8 million.  •   Average aftermarket parts sales prices increased by $69.5 million reflecting improved price realization from  improved market demand. •   Average aftermarket parts costs increased $41.8 million from higher material costs.  •   The currency translation effect on sales and cost of sales primarily reflects a stronger euro. Truck SG&A was $415.1 million in 2011 compared to $368.3 million in 2010. The higher spending is due to higher  salaries and related expenses of $51.4 million (including $10.1 million from the effect of foreign currencies) to  support higher levels of business activity. As a percentage of sales, SG&A decreased to 2.7% in 2011 from 4.0% in  2010 due to higher sales volumes and ongoing cost control. Truck R&D expenditures increased to $287.5 million in 2011 from $238.2 million in 2010. The higher spending in  2011 reflects increased new product development activities, primarily for new truck products for North America  and Europe and $8.3 million from higher foreign currencies, primarily the euro. Truck gross margins in 2011 reflect the benefits of higher market demand and increased absorption of fixed costs  resulting from higher truck production. Aftermarket parts gross margins in 2011 benefited from higher price  realization from improved market demand. Net sales and revenues and gross margins for truck units and aftermarket parts are provided below. The aftermarket  parts gross margin includes direct revenues and costs, but excludes certain Truck segment costs. ($ in millions) Year ended December 31, 2011  2010  %  CHANGE Truck net sales and revenues: Trucks  $ 12,630.7 $ 7,042.9  79 Aftermarket parts  2,577.0 2,194.4  17 $ 15,207.7 $ 9,237.3  65 Gross margin: Trucks  $ 1,072.8 $ 366.1  193 Aftermarket parts  892.9 745.7  20 $ 1,965.7 $ 1,111.8  77 Gross margin %: Trucks  8.5% 5.2%  Aftermarket parts  34.7% 34.0%  12.9% 12.0% 