operating/performance statistics railroad performance measures reported to the aar , as well as other performance measures , are included in the table below : 2010 2009 2008 % (  % ) change 2010 v 2009 % (  % ) change 2009 v 2008 .
what is the mathematical range for average train speed ( mph ) for 2008-2010?
average train speed 2013 average train speed is calculated by dividing train miles by hours operated on our main lines between terminals . maintenance activities and weather disruptions , combined with higher volume levels , led to a 4% ( 4 % ) decrease in average train speed in 2010 compared to a record set in 2009 . overall , we continued operating a fluid and efficient network during the year . lower volume levels , ongoing network management initiatives , and productivity improvements contributed to a 16% ( 16 % ) improvement in average train speed in 2009 compared to 2008 . 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 increased 2% ( 2 % ) in 2010 compared to 2009 , driven in part by our network plan to increase the length of numerous trains to improve overall efficiency , which resulted in higher terminal dwell time for some cars . average terminal dwell time improved slightly in 2009 compared to 2008 due to lower volume levels combined with initiatives to expedite delivering rail cars to our interchange partners and customers . average rail car inventory 2013 average rail car inventory is the daily average number of rail cars on our lines , including rail cars in storage . lower average rail car inventory reduces congestion in our yards and sidings , which increases train speed , reduces average terminal dwell time , and improves rail car utilization . average rail car inventory decreased 3% ( 3 % ) in 2010 compared to 2009 , while we handled 13% ( 13 % ) increases in carloads during the period compared to 2009 . we maintained more freight cars off-line and retired a number of old freight cars , which drove the decreases . average rail car inventory decreased 6% ( 6 % ) in 2009 compared to 2008 driven by a 16% ( 16 % ) decrease in volume . in addition , as carloads decreased , we stored more freight cars off-line . 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 and revenue-ton-miles increased 10% ( 10 % ) and 9% ( 9 % ) in 2010 compared to 2009 due to a 13% ( 13 % ) increase in carloads . commodity mix changes ( notably automotive shipments ) drove the variance in year-over-year growth between gross ton-miles , revenue ton-miles and carloads . gross and revenue ton-miles decreased 17% ( 17 % ) and 15% ( 15 % ) in 2009 compared to 2008 due to a 16% ( 16 % ) decrease in carloads . commodity mix changes ( notably automotive shipments , which were 30% ( 30 % ) lower in 2009 versus 2008 ) drove the difference in declines between gross ton-miles and revenue ton- miles . operating ratio 2013 operating ratio is defined as our operating expenses as a percentage of operating revenue . our operating ratio improved 5.5 points to 70.6% ( 70.6 % ) in 2010 and 1.3 points to 76.1% ( 76.1 % ) in 2009 . efficiently leveraging volume increases , core pricing gains , and productivity initiatives drove the improvement in 2010 and more than offset the impact of higher fuel prices during the year . core pricing gains , lower fuel prices , network management initiatives , and improved productivity drove the improvement in 2009 and more than offset the 16% ( 16 % ) volume decline . employees 2013 employee levels were down 1% ( 1 % ) in 2010 compared to 2009 despite a 13% ( 13 % ) increase in volume levels . we leveraged the additional volumes through network efficiencies and other productivity initiatives . in addition , we successfully managed the growth of our full-time-equivalent train and engine force levels at a rate less than half of our carload growth in 2010 . all other operating functions and .