OECD, Revenue Statistics 1965-2008 (2009 ed.):

The overall tax-to-GDP ratio in OECD countries was 35.8% in 2007, the latest year for which complete figures are available. … Out of 26 OECD countries that have provided provisional figures for 2008, tax-to-GDP ratios fell in 17 and rose in 9. … Denmark has the highest tax-to-GDP ratio among OECD countries in 2008 (48.3%), closely followed by Sweden (47.1%). …  Mexico and Turkey have the lowest tax-to-GDP ratios among OECD countries. Mexico collected taxes equivalent to 21.1% of GDP in 2008 … Turkey’s tax-to-GDP ratio was 23.5% in 2008.