pre_text: other operating/performance and financial statistics we report key railroad performance measures weekly to the association of american railroads ( aar ) , including carloads , average daily inventory of rail cars on our system , average train speed , and average terminal dwell time . we provide this data on our website at www.up.com/investors/reports/index.shtml . operating/performance statistics included in the table below are railroad performance measures reported to the aar : 2008 2007 2006 % (  % ) change 2008 v 2007 % (  % ) change 2007 v 2006 .
post_text: average train speed 2013 average train speed is calculated by dividing train miles by hours operated on our main lines between terminals . ongoing network management initiatives , productivity improvements , and lower volume levels contributed to 8% ( 8 % ) and 2% ( 2 % ) improvements in average train speed in 2008 and 2007 , respectively . average terminal dwell time 2013 average terminal dwell time is the average time that a rail car spends at our terminals . lower average terminal dwell time improves asset utilization and service . average terminal dwell time improved 1% ( 1 % ) and 8% ( 8 % ) in 2008 and 2007 , respectively . lower volumes combined with initiatives to more timely deliver rail cars to our interchange partners and customers improved dwell time in both periods . gross and revenue ton-miles 2013 gross ton-miles are calculated by multiplying the weight of loaded and empty freight cars by the number of miles hauled . revenue ton-miles are calculated by multiplying the weight of freight by the number of tariff miles . gross ton-miles decreased 3% ( 3 % ) , while revenue ton-miles were flat in 2008 compared to 2007 with commodity mix changes ( notably autos and coal ) explaining the variance in year over year growth between the two metrics . in 2007 , revenue ton-miles declined 1% ( 1 % ) in relation to the 1% ( 1 % ) reduction in carloadings compared to 2006 . gross ton-miles decreased 2% ( 2 % ) in 2007 driven by a mix shift in freight shipments . operating ratio 2013 operating ratio is defined as our operating expenses as a percentage of operating revenue . our operating ratios improved 2.0 points to 77.3% ( 77.3 % ) in 2008 and 2.2 points to 79.3% ( 79.3 % ) in 2007 . price increases , fuel cost recoveries , network management initiatives , and improved productivity more than offset the impact of higher fuel prices . employees 2013 productivity initiatives and lower volumes reduced employee levels throughout the company in 2008 versus 2007 . fewer train and engine personnel due to improved network productivity and 5% ( 5 % ) lower volume drove the change while productivity initiatives within the support organizations also contributed to a lower full-time equivalent force level . lower employee levels in 2007 versus 2006 .
table: <i>2008</i> <i>2007</i> <i>2006</i> <i>% Change 2008 v 2007</i> <i>% Change 2007 v 2006</i>
table: Average train speed (miles per hour) 23.5 21.8 21.4 8 % 2 %
table: Average terminal dwell time (hours) 24.9 25.1 27.2 (1)% (8)%
table: Average rail car inventory (thousands) 300.7 309.9 321.6 (3)% (4)%
table: Gross ton-miles (billions) 1,020.4 1,052.3 1,072.5 (3)% (2)%
table: Revenue ton-miles (billions) 562.6 561.8 565.2 - (1)%
table: Operating ratio 77.3 79.3 81.5 2.0 pt 2.2 pt
table: Employees (average) 48,242 50,089 50,739 (4)% (1)%
table: Customer satisfaction index 83 79 72 4 pt 7 pt
question: Analyse this data from a financial earnings document. based on the operating/performance statistics what was the average operating ratio from 2006 to 2008