| 年份 | 美国 |
|---|---|
| 2023 | - |
| 2022 | 27.6672 |
| 2021 | 28.0325 |
| 2020 | 28.3633 |
| 2019 | 28.6165 |
| 2018 | 28.7877 |
| 2017 | 28.9262 |
| 2016 | 28.9844 |
| 2015 | 29.0729 |
| 2014 | 29.234 |
| 2013 | 29.3471 |
| 2012 | 29.4163 |
| 2011 | 29.4745 |
| 2010 | 29.6057 |
| 2009 | 29.8 |
| 2008 | 29.9668 |
| 2007 | 30.1445 |
| 2006 | 30.405 |
| 2005 | 30.7493 |
| 2004 | 31.1239 |
| 2003 | 31.4589 |
| 2002 | 31.7547 |
| 2001 | 32.0477 |
| 2000 | 32.3368 |
| 1999 | 32.5757 |
| 1998 | 32.737 |
| 1997 | 32.8815 |
| 1996 | 33.0452 |
| 1995 | 33.2082 |
| 1994 | 33.2965 |
| 1993 | 33.2632 |
| 1992 | 33.1129 |
| 1991 | 32.8597 |
| 1990 | 32.5832 |
| 1989 | 32.3508 |
| 1988 | 32.1236 |
| 1987 | 31.9707 |
| 1986 | 32.0441 |
| 1985 | 32.3023 |
| 1984 | 32.5857 |
| 1983 | 32.8746 |
| 1982 | 33.1749 |
| 1981 | 33.4793 |
| 1980 | 33.9215 |
| 1979 | 34.5845 |
| 1978 | 35.4618 |
| 1977 | 36.4884 |
| 1976 | 37.6548 |
| 1975 | 38.9688 |
| 1974 | 40.3203 |
| 1973 | 41.7057 |
| 1972 | 43.1422 |
| 1971 | 44.4903 |
| 1970 | 45.6832 |
| 1969 | 46.8245 |
| 1968 | 47.9957 |
| 1967 | 49.0182 |
| 1966 | 50.2824 |
| 1965 | 51.0826 |
| 1964 | 50.9526 |
| 1963 | 50.9798 |
| 1962 | 51.3489 |
| 1961 | 51.7115 |
| 1960 | 51.3974 |