[Sohcahtoa82]

There's a sweet spot when it comes to power and efficiency. A car that only has enough power to go 70 mph will likely be very inefficient at it, not to mention that it would take a very long time to get there. On the other side of the spectrum, a 600+ horsepower monster than can go 200+ mph is pretty inefficient at 70 mph, too.

But somewhere in the 150-200 horsepower range will end up with a car that is very efficient at 70 mph, and a top speed around 130-150 mph depending on gearing an aerodynamics.

Of course, rather than designing cars with lower power, they could simply add a governor, so the car computer does not allow travel above a certain speed, but that would be an incredibly unpopular option, even among people that don't go significantly over the limit.