adjusted net income of $ 4.6 billion translated into adjusted earnings of $ 5.79 per diluted share , a best- ever performance . f0b7 freight revenues 2013 our freight revenues increased 7% ( 7 % ) year-over-year to $ 19.8 billion driven by volume growth of 2% ( 2 % ) , higher fuel surcharge revenue , and core pricing gains . growth in frac sand , coal , and intermodal shipments more than offset declines in grain , crude oil , finished vehicles , and rock shipments . f0b7 fuel prices 2013 our average price of diesel fuel in 2017 was $ 1.81 per gallon , an increase of 22% ( 22 % ) from 2016 , as both crude oil and conversion spreads between crude oil and diesel increased in 2017 . the higher price resulted in increased operating expenses of $ 334 million ( excluding any impact from year- over-year volume growth ) . gross-ton miles increased 5% ( 5 % ) , which also drove higher fuel expense . our fuel consumption rate , computed as gallons of fuel consumed divided by gross ton-miles in thousands , improved 2% ( 2 % ) . f0b7 free cash flow 2013 cash generated by operating activities totaled $ 7.2 billion , yielding free cash flow of $ 2.2 billion after reductions of $ 3.1 billion for cash used in investing activities and $ 2 billion in dividends , which included a 10% ( 10 % ) increase in our quarterly dividend per share from $ 0.605 to $ 0.665 declared and paid in the fourth quarter of 2017 . free cash flow is defined as cash provided by operating activities less cash used in investing activities and dividends paid . free cash flow is not considered a financial measure under gaap by sec regulation g and item 10 of sec regulation s-k and may not be defined and calculated by other companies in the same manner . we believe free cash flow is important to management and investors in evaluating our financial performance and measures our ability to generate cash without additional external financings . free cash flow should be considered in addition to , rather than as a substitute for , cash provided by operating activities . the following table reconciles cash provided by operating activities ( gaap measure ) to free cash flow ( non-gaap measure ) : .
what was the percentage change in free cash flow from 2015 to 2016?
2018 outlook f0b7 safety 2013 operating a safe railroad benefits all our constituents : our employees , customers , shareholders and the communities we serve . we will continue using a multi-faceted approach to safety , utilizing technology , risk assessment , training and employee engagement , quality control , and targeted capital investments . we will continue using and expanding the deployment of total safety culture and courage to care throughout our operations , which allows us to identify and implement best practices for employee and operational safety . we will continue our efforts to increase detection of rail defects ; improve or close crossings ; and educate the public and law enforcement agencies about crossing safety through a combination of our own programs ( including risk assessment strategies ) , industry programs and local community activities across our network . f0b7 network operations 2013 in 2018 , we will continue to align resources with customer demand , maintain an efficient network , and ensure surge capability of our assets . f0b7 fuel prices 2013 fuel price projections for crude oil and natural gas continue to fluctuate in the current environment . we again could see volatile fuel prices during the year , as they are sensitive to global and u.s . domestic demand , refining capacity , geopolitical events , weather conditions and other factors . as prices fluctuate , there will be a timing impact on earnings , as our fuel surcharge programs trail increases or decreases in fuel price by approximately two months . lower fuel prices could have a positive impact on the economy by increasing consumer discretionary spending that potentially could increase demand for various consumer products that we transport . alternatively , lower fuel prices could likely have a negative impact on other commodities such as coal and domestic drilling-related shipments. .