Range decreases exponentially with velocity, because the force of drag (which is the primary resistance to a vehicle moving at high speeds) is
proportional to the square of velocity.
At low speeds, the main limitations are electrical needs to keep the vehicle running (heat, cooling, etc) and different kinds of energy loss due to friction (drivetrain, tires).
EPA highway efficiency is rated at 55mph, so manufacturers generally try to maximize efficiency at that speed.
Here's an old table comparing Tesla's various tire options for the Model 3, but also includes efficiency at various speeds:
View attachment 7474
55mph is
much better than 65mph, which is
much better than 75mph. The sweet spot would probably be about 15-20mph; you could probably go at 15-20mph for almost double the EPA rated range, I'd bet! But if you did that you'd get murdered by other drivers.
If you're on a long road trip, I'd stick to 65-70; you won't be in the leftmost lane, and you'll occasionally be getting passed, but hey, you'll have the range you want (although I have to stop every few hours to pee/walk around anyway, so why not just coincide that with charging stops). Otherwise, enjoy going 75, 80, 85, whatever... just know that with each increase in velocity you lose range by the difference in that velocity
squared (simplifying, of course, but all other things being equal).
This is
also why the extremely low Cd of the Lucid Air is so important; it is by far the biggest impact on range.