I have a 2013 S, no autopilot, so the dashboard shows energy usage in real-time. I've found that rated range is very easy to achieve when I pay attention to kw usage instead mph. My rule of thumb was to try to cruise around 20kw as much as possible. Going up hill, try to stay at 40kw max. Going on a steep downhill, keep it in the green so that we're generating.
As for real-world speed, that translated to 70mph cruising most of the time. On big hills (climbing the grapevine, for example) I would have to slow down a lot and 50-60kw would be necessary and still get passed by everyone. On downhills I'd try to keep it at 75mph or less just for the regen.
So with autopilot and the newer cars, Tesla's UI is trying to stay away from real-time kw and just let you drive the car. If you're into hypermiling, though, the energy usage is useful and I sort of miss that data on the newer S, X and 3.
The Model 3 does show real-time energy usage, but it's just a bar across the speedometer section of the screen without any indication of kw used/generated. Still, even without the actual numbers, you can make a mental note of where the bar is during normal highway cruising on a flat highway at 70mph and keep it in mind as a target when trying to hypermile.
Anyway, my point is, like others have said, if you're going uphill, just a 5mph change in speed has a significant effect on energy usage. I've found that 70-75mph on the flats, 75mph when going downhill, and 65 when going on a significant uphill will get you rated range most of the time.
This is just my OCD, too. Normal driving for me without hypermiling has always been just a bit under rated range without paying attention. I've only had the Model 3 a few days now, but I expect it'll be about the same, if not better.
The Model 3 is certainly more efficient: I've been averaging 250wh/mile but haven't done a road trip yet.