Going to need to revise my answer, a few days ago I reduced the highway cruising speed for my commute from 55mph to 50mph. Highway is only ~50% of the drive, but it the improvement appears to be approximately from 73mpg to 79mpg, which may well exceed anything else I've done, or even all of it.
I go back and forth on how much my time is worth. I expect the next time I take a road trip I'll still be driving 65, but right now I'm of the opinion that I'm not really missing that extra minute or two in the morning and afternoon. And, I'm saving about 6 cents per day extra.
