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Agreed, mileage ratings are the product of the marketing department. If the estimate is made with integrity, it can provide a consumer with no knowledge, an elementary or crude understanding of the antennas performance. There is no uniform standard to make mileage claims, so one needs to consider the credibility of the source and then consider if their application is similar to the hypothetical (and unpublished) conditions of the estimate.
Gain expressed in dB is the product of the engineering department. When measured or calculated accurately, someone with a bit of understanding of the science can predict with much greater reliability, the expected performance of the installed product.
Of the two manufacturers mentioned above, both manufacturers offer credible gain figures. Winegard uses units of dBd (decibels referenced to a tuned dipole), Antennas Direct uses units of dBi (decibels referenced to an isotropic radiator). Both units of measure have merit and are valid. To convert dBd to dBi, use dBd+2.15=dBi and conversely, dBi-2.15=dBd.
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