From my experience they get more efficient over 90 mph, I only every go that fast when I have a copilot to help me navigate and help me check lanes before passing. Anytime you let off the accelerator they recharge the battery, its extremely funny to watch the estimated arrival percentage to go up when you are crusing over 100 and occasionally taking your foot off the accelerator. Such a fun driving machine, i just wish Elon wasnt such a cunt.