advance the timing as far as possible, just to the point where you start to get spark knock. that point, in general, will give the best economy AND best power. just be careful, because you may not hear continuous spark knock at highway speeds. also, the engine will run hotter the more you advance the timing. if you're really into it, you could switch permanently to premium fuel, and set it up for that. the higher octane is there to prevent the knock and you could potentially get even more power and economy with that fuel, and timing adjusted to take advantage of it. it may be best to watch your economy every fillup to get a running average, and then try playing with changes, to see if there is any measureable difference.