This new report includes a 2010 study from New York that had men self-report, then actually measured them and compared.
Here's what they found: Black men reported 7.9 inches and measured 6.9 (difference of -1.0 inches)... Hispanic men reported 7.7 and measured at 6.5 (-1.2)... Asian men reported 6.7 and measured 5.3 (-1.4)... and white men lied the most, reporting 7.8 and measuring at 6.1 (-1.7).