Really depends, if I blitz along at 80, then yeah. If I stick to the speed limit, it’s not a bad estimate, on mostly slow local roads, we get substantially better. It’s a great demonstration of drag’s nonlinear nature (the cube of the speed?).
I've always been under the impression 55 was the optimum speed for (ICE) fuel efficiency.
Does this change at all for electrics (given regenerative braking)?
To answer your regenerative braking question: braking is inefficient by default, so just avoid tapping the brakes in situations where you could coast.
With regenerative braking, you'll never get back 100% the energy because of thermodynamics. There's always wasted energy any time energy is moved, transformed, stored, etc.
That's because my car never consumes less than around 0.3-0.4 gallons per hour when the engine is running.
It would be slightly higher if you run the AC on high, increasing total consumption by ~10% and moving the dial a bit.