Jet fuel is our largest energy source. As a result, it is one of our largest expenses and our single greatest source of emissions.
Because of the direct relationship between our fuel consumption and the cost associated with that consumption, we recognize the importance of pursuing multiple fuel efficiency initiatives. These initiatives help us in our effort to reduce fuel consumption and emissions, and maintain a low-cost advantage in the airline industry. Since 2002, Southwest has invested more than $620 million in fuel efficiency improvements, routinely performing engine washes, utilizing ground electric power while aircraft are parked at airport gates, and reducing ground idle speeds for aircraft in our fleet.
While we continued to invest in fuel efficiency improvements, in March 2019, we were required to remove our 34 Boeing 737 MAX 8 aircraft from service. These are the most fuel-efficient aircraft in our fleet, and their removal from service impacted our overall fuel efficiency, resulting in a slight decline in our fleet-wide fuel efficiency during 2019. For more information about the 737 MAX 8, click here.
See Footnote: 8888) Baseline year for fuel efficiency improvement on an RTM/gal basis has been adjusted from 2005 to 2000. 2019 fuel efficiency improvement using 2005 as baseline is 31.9%., 8989) A revenue ton mile (RTM) is one ton of revenue traffic (passenger and cargo) transported one mile.