our refining and wholesale marketing gross margin is the difference between the prices of refined products sold and the costs of crude oil and other charge and blendstocks refined , including the costs to transport these inputs to our refineries , the costs of purchased products and manufacturing expenses , including depreciation . the crack spread is a measure of the difference between market prices for refined products and crude oil , commonly used by the industry as a proxy for the refining margin . crack spreads can fluctuate significantly , particularly when prices of refined products do not move in the same relationship as the cost of crude oil . as a performance benchmark and a comparison with other industry participants , we calculate midwest ( chicago ) and u.s . gulf coast crack spreads that we feel most closely track our operations and slate of products . posted light louisiana sweet ( 201clls 201d ) prices and a 6-3-2-1 ratio of products ( 6 barrels of crude oil producing 3 barrels of gasoline , 2 barrels of distillate and 1 barrel of residual fuel ) are used for the crack spread calculation . our refineries can process significant amounts of sour crude oil which typically can be purchased at a discount to sweet crude oil . the amount of this discount , the sweet/sour differential , can vary significantly causing our refining and wholesale marketing gross margin to differ from the crack spreads which are based upon sweet crude . in general , a larger sweet/sour differential will enhance our refining and wholesale marketing gross margin . in 2009 , the sweet/sour differential narrowed , due to a variety of worldwide economic and petroleum industry related factors , primarily related to lower hydrocarbon demand . sour crude accounted for 50 percent , 52 percent and 54 percent of our crude oil processed in 2009 , 2008 and 2007 . the following table lists calculated average crack spreads for the midwest ( chicago ) and gulf coast markets and the sweet/sour differential for the past three years . ( dollars per barrel ) 2009 2008 2007 .
by what percentage did the average crack spread for the midwest ( chicago ) decrease from 2007 to 2009?
sweet/sour differential ( a ) $ 5.82 $ 11.99 $ 11.59 ( a ) calculated using the following mix of crude types as compared to lls. : 15% ( 15 % ) arab light , 20% ( 20 % ) kuwait , 10% ( 10 % ) maya , 15% ( 15 % ) western canadian select , 40% ( 40 % ) mars . in addition to the market changes indicated by the crack spreads and sweet/sour differential , our refining and wholesale marketing gross margin is impacted by factors such as : 2022 the types of crude oil and other charge and blendstocks processed , 2022 the selling prices realized for refined products , 2022 the impact of commodity derivative instruments used to manage price risk , 2022 the cost of products purchased for resale , and 2022 changes in manufacturing costs , which include depreciation . manufacturing costs are primarily driven by the cost of energy used by our refineries and the level of maintenance costs . planned turnaround and major maintenance activities were completed at our catlettsburg , garyville , and robinson refineries in 2009 . we performed turnaround and major maintenance activities at our robinson , catlettsburg , garyville and canton refineries in 2008 and at our catlettsburg , robinson and st . paul park refineries in 2007 . our retail marketing gross margin for gasoline and distillates , which is the difference between the ultimate price paid by consumers and the cost of refined products , including secondary transportation and consumer excise taxes , also impacts rm&t segment profitability . there are numerous factors including local competition , seasonal demand fluctuations , the available wholesale supply , the level of economic activity in our marketing areas and weather conditions that impact gasoline and distillate demand throughout the year . refined product demand increased for several years until 2008 when it decreased due to the combination of significant increases in retail petroleum prices , a broad slowdown in general economic activity , and the impact of increased ethanol blending into gasoline . in 2009 refined product demand continued to decline . for our marketing area , we estimate a gasoline demand decline of about one percent and a distillate demand decline of about 12 percent from 2008 levels . market demand declines for gasoline and distillates generally reduce the product margin we can realize . we also estimate gasoline and distillate demand in our marketing area decreased about three percent in 2008 compared to 2007 levels . the gross margin on merchandise sold at retail outlets has been historically less volatile. .