As Leftie said, the slower the better but of course you don't to be the only one going 50 or 55 if everyone else is doing 70mph. I've noticed a much faster drop in charge going over 60 so if you can do 55 that would probably be best if traffic allows, 60 if not. With the Leaf(and probably all EVs) side roads where the speed is slower is better than freeway speeds, even if you have to stop for signs or lights.
EVs really shine in stop and go traffic, getting better than 70 with no traffic, when driving my EV it's the one thing positive to say about slow traffic
70 is a real range killer but if I have plenty of charge or a short distance the Leaf handles 70+ just fine, just kills range.