Source : Free On-Line Dictionary of Computing

Your mileage may vary
     
        (Standard disclaimer attached to EPA mileage ratings by
        American car manufacturers) A ritual warning often found in
        Unix {freeware} distributions and elsewhere.  Translates
        roughly as "Hey, I tried to write this portably, but who
        *knows* what'll happen on your system?"
     
        [{Jargon File}]
     
        (1994-11-10)