This paper discusses the effects on the backscattered power of placing RFID tags on vehicular windshields. The first analyzed effect is the mismatch produced between the antenna of an RFID tag and its chip due to the changes of the surrounding medium of the RFID tag. The second effect is produced by the gain penalty which quantifies the variation of the tag effective gain due to the alteration of the tag near field. Both effects are calculated using full wave simulations for three tags and included in the power link budget to compare the estimation of the backscattered power with measurements. Results of commercial tags on polarized windshield glasses show that the power budget is mainly affected by the mismatch effect and can be responsible of important power losses.