The most efficient speed for virtually all vehicles is about 50-55 mph, and that’s probably going to be a more pronounced difference on something that’s less aerodynamic. 65-70 doesn’t tend to be too bad though, no.

Absolutely not for electric, which is far more efficient at lower speeds, unlike gas which has a minimum burn even at idle and downhill.

You're thinking of ICE vehicles, which have efficient torque at certain RPMs, and don't recover energy when they brake, so traffic is even more inefficient. Electric vehicles have all the torque, all the time, and do recover energy, so for them it's more about wind resistance, which increases with speed.