A minor decrease between 1966 and 1973 (from 13.5 mpg to 12.9 mpg). A modest increase between 1973 and 1991 (from 12.9 mpg to 19.6 mpg). No change between 1991 and 2004 (19.6 mpg for both years). A modest increase between 2004 and 2008 (from 19.6 mpg to 21.8 mpg).