i don't know... haven't had a working cluster for a minute so i don't have a good number as far as mpg goes...
you can advance the timing quite a bit at idle and little throttle but then get all kinds of valve chatter when you get on the throttle at that way advanced setting. Well, taking that tid bit, when you are on the interstate, just cruisin, you aren't under a heavier load as say accelerating, so you can be real light on the throttle. The thought was that you could set the timing really advanced, and make a little more power, resulting in a little less throttle, meaning better mpg to maintain the same speeds.
We were more or less talking in more of a theoretical sense.
I'm sure in a practical sense, the gain wouldn't be worth the hassle...