https://www.epa.gov/fueleconomy/basic-information-fuel-economy-labeling#:~:text=Updates to Fuel Economy Test Methods and Calculations&text=The 2008 changes were broad,small number by 2 MPG.

EPA periodically updates its methodology to account for changes in vehicle technologies, driver behavior, and/or driving conditions. The 2008 changes were broad revisions to the entire methodology that lowered the fuel economy estimates for all vehicles. The updates for 2017 will reduce some fuel economy estimates by 1 mile per gallon (MPG) and a small number by 2 MPG.