If it was going to rev 5-6,000 on the interstate they could just use a motorcycle engine, I'm sure this isn't the issue. As for dragging around a jail cell (iron cage) with you everywhere, if we invested more in NASA and stopped cutting their budget in half every year, (thanks W), I'm sure we could find something more liberating than steel, more practical than glass and carbon fibers, and lighter than aramids.
We could just cut the NASA budget completely and use the money right here and solve a few or many problems too . or Just let Bruce drive the car he wants and hypermile it to prove it can be done . Kind of similar huh ? Space travel and exploration vs hypermiling a brick ...Einstein was right - its *all* relative
The problem isn't that the engine can't run that high (They might as well be motorcycle engines, what's the beat rev to, 9k?), it's that you'll get bad fuel economy on the highway with their supershort gearing.