We generate simulations of the CMB temperature field as observed by the WMAP satellite , taking into account the detailed shape of the asymmetric beams and scanning strategy of the experiment , and use these to re-estimate the WMAP beam transfer functions . This method avoids the need of artificially symmetrizing the beams , as done in the baseline WMAP approach , and instead measures the total convolution effect by direct simulation . We find noticeable differences with respect to the nominal transfer functions . For instance , the nominal V1 beam under-estimates the full beam convolution by \sim 0 .5 % at \ell = 500 and \sim 1.0 % at \ell = 800 . Similar differences are seen for other DA ’ s . This in turn implies that the high- \ell power spectrum is biased low by 1 - 2 % , effectively tilting the spectrum slightly . Re-estimating cosmological parameters we find that the spectral index of scalar perturbations is n _ { \textrm { s } } = 0.969 \pm 0.014 after correcting for this effect , corresponding to a positive shift of 0.3 \sigma compared to the previously released WMAP results . Our CMB sky simulations are made publicly available , and can be used for general studies of asymmetric beam effects in the WMAP data .