The relationship between gallons used over a given distance and miles per gallon (mpg) is not linear. For example, replacing a truck that gets 10 mpg for a new one that gets 15 mpg will save 33 gallons of fuel for every 1,000 miles driven. In contrast, replacing a 30-mpg car with a new car that gets 35 mpg will save 5 gallons of fuel for every 1,000 miles driven.
Note: Each category on the horizontal axis shows a five-mile per gallon improvement in fuel economy.
Oak Ridge National Laboratory, Transportation Energy Data Book: Edition 37, ORNL/TM-2018/198, January 2019, Figure 4.3.
[Original source: U.S. Department of Energy fuel economy data.]