A '55 Caddy Eldorado will get 10.5mpg highway according to these folks: http://www.automobile-catalog.com/car/1955/326075/cadillac_e...
They're cool cars but today's $70k luxury cars are faster, stronger, more reliable, and functionally better. The old Caddies do look damn good, but I'll take a DOHC versus an OHV any day.
My point is that the technology has clearly progressed. To your point, maybe not exponentially, true. But there has definitely been a whole heap of engineering work over 60+ years that has resulted in unequivocally better, more reliable, markedly different vehicles.
All of the other stuff - better acceleration, air bags, automatic transmissions, A/C - those are refinements on a capability that fundamentally didn't change. They both drive on roads at around 60 mph at approximately 25 mpg.