Driving speed does make a big difference, next to wind, to which I think the limit minus 10% is a good option, and not too slow for the rest of the traffic. Driving about 60 in a 65 would seem to be fine, and should not feel too slow either. My best tank was at about 61, and I got over 38 mpg!

2008 Ford Escape Hybrid - Adopted 7-7-07