Winegard and Antennacraft are good about publishing technical performance specifications in a format that can be checked with calibrated test equipment in an independent environment. Antennas Direct also offers some similar information though not quite as much detail. Channel Master is the most tight lipped of trusted manufactures and simply offers 'advertising department' grade information.
Example of what I prefer to see:
http://www.winegard.com/kbase/upload/HD7084P.pdf
Mileage claims are not a standard / repeatable measurement system. They are sometimes helpful but purposely vague as marketing speak can so often be.