The object of this work was to identify the origin of differences between sound levels measured on two track types used for standardized motor vehicle passby noise tests: i.e., the ISO and SAE surfaces. The measured differences could result either from the effect of the road surface on tire/road interaction noise generation, or from surface sound absorption differences. Near-field intensity data measured at the tire/road interface on vehicles in motion were first examined to establish the correlation between surface type and source spectrum and level. Source levels were approximately 1 dB higher at SAE sites than at ISO sites. Two-microphone methods were then used to estimate the acoustic impedance of both the ISO and SAE surfaces. The surface impedances were used together with measured tire source spectra and sound propagation theories to calculate the passby level variability that could be attributed to variations in test site acoustic impedance. It was found that differences in propagation transfer functions for the ISO and SAE surfaces were on the order of only 0.3 dB. Thus it was concluded that the differences in passby levels measured at ISO and SAE sites resulted primarily from differences in tire/road interaction noise generation.