Your figures are incorrect:
Voting age population 2008 = 230,872,030 http://elections.gmu.edu/Turnout_2008G.html
Voting age population 2012 = 240,926,957 http://elections.gmu.edu/Turnout_2012G.html
It took me a while to find out, and I had almost decided that all my other research must have been it, when I realized that I had not considered the possibility of your numbers being wrong.
It looks like they were.
Did you look at the link? You're saying the info from that link is wrong? Wonder why the big difference?
If the wiki link has wrong information, I'd like to know.
Election Voting Age Population (VAP)
(thousands)[1] Turnout
(thousands)[1] % Turnout of VAP[2][1]
1789
1792
1796
1800
1804
1808
1812
1816
1820
1824
1828 57.6%
1832 55.4%
1836 57.8%
1840 80.2%
1844 78.9%
1848 72.7%
1852 69.6%
1856 78.9%
1860 81.2%
1864 73.8%
1868 78.1%
1872 71.3%
1876 81.8%
1880 79.4%
1884 77.5%
1888 79.3%
1892 74.7%
1896 79.3%
1900 73.2%
1904 65.2%
1908 65.4%
1912 58.8%
1916 61.6%
1920 49.2%
1924 48.9%
1928 56.9%
1932 75,768 39,817 52.6%
1936 80,174 45,647 56.9%
1940 84,728 49,815 58.8%
1944 85,654 48,026 56.1%
1948 95,573 48,834 51.1%
1952 99,929 61,552 61.6%
1956 104,515 62,027 59.3%
1960 109,672 68,836 62.8%
1964 114,090 70,098 61.4%
1968 120,285 73,027 60.7%
1972 140,777 77,625 55.1%
1976 152,308 81,603 53.6%
1980 163,945 86,497 52.8%
1984 173,995 92,655 53.3%
1988 181,956 91,587 50.3%
1992 189,493 104,600 55.2%
1996 196,789 96,390 49.0%
2000 209,787 105,594 50.3%
2004 219,553 122,349 55.7%
2008 229,945 131,407 57.1%
2012 211,731 121,745 57.5%