The gains made by the Tesla S are mainly due to the form factor - it is uniquely advantaged by being a large all-electric sedan.

Longer vehicles have a significant aerodynamic advantage, because they can more closely approximate a perfect airfoil than a shorter vehicle. The Tesla S has a very smooth underbody because of the flat battery unit and because it requires no exhaust system. Air intakes are kept to a minimum, because it does not need to dissipate the copious waste heat produced by a combustion engine.

I don't mean to detract from the marvellous engineering efforts by the Tesla team, but the Model S is in a class of one. Large sedans from mainstream manufacturers come very close to the aerodynamic performance of the Model S, but the use of internal combustion puts them at an inherent disadvantage. Competing all-electric cars are predominantly cheaper and shorter vehicles, designed for the European city car market. The high price of the Model S allows Tesla to use a lot of trick aero features, like the bladed mirror mountings, active inlet vents and retractible door handles.

I have no doubt that the traditional manufacturers could match the aero performance of the Model S if they were to produce a car with a similar form factor and price; The Mercedes CLA BlueEfficiency exceeds the aerodynamic performance of the Tesla S, as one of the few large and expensive cars that have been optimised primarily for efficiency. Volkswagen manage a drag coefficient of just 0.159 with the XL1, but it costs $146,000.

I like your comment but this seems like a total non sequitur? Are you saying that if it's not a multiway tie for the best in something, then that implies there "must be quite some room for improvement in that area"? Or how do you get to this conclusion?

It had a low drag coefficient, but there wasn't much of a practical design to maintain: "the car is so tall that a stiff crosswind could send it careening to the side."