Cars in the seventies also tended to be smaller, meaning there was a smaller cross-sectional area on the vehicle. I think we have more efficient engines now, but I think the differences between larger more aerodynamic vehicles versus smaller less aerodynamic vehicles is negligible. If someone has numbers, then I'm happy to change my mind.
Cars today are much more aerodynamic than they were 50 years ago. That almost certainly changes the "economics" of such a change drastically.