Naturally aspired engines (i.e. not electric or turbo) will get a decrease in power and efficiently correlating with air pressure drop, just like driving at altitude. Also, removing air resistance won't on its own won't allow normal cars to go that much faster efficiently.

**Keep in mind I haven't yet listened to the podcast for context, **but on flat terrain, air drag is by far the largest resistance factor (>90%), and so anything that reduces aerodynamic drag will result in an increase in speed for same power, or reduced power demand for the same speed. If you remove air resistance (while not hampering ability to generate power), a vehicle will be able to travel a huge amount faster for the same power output.

One thing to point out is that the numbers Jay gave (speed vs economy) are averages. The more aerodynamic the vehicle, and the smaller the frontal area, the faster you can go before aerodynamic drag starts to increase quadraritically or worse with speed. (Boxier shapes have a worse-than-quadratic ramp-up in drag.) It is an "empirical law" meaning most aerodynamic stuff that we makes experiences approximately quadratic increased in drag at the speeds we typically use them.

Putting aside any funkiness with the relationship between CdA* and

Reynolds number**, then the function of drag with air velocity is a quadratic equation no matter the speed of the car and wind, nor the shape of the vehicle. There is no speed it cuts in or cuts out (unless and until the relationship between CdA and Reynolds number changes - which is the sort of thing that occurs over orders of magnitude changes in velocity).

All that happens with vehicles with a less streamlined shaped is they have a higher CdA. As a result at the same air velocity (ceteris paribus) they present a greater drag, and hence require more power to sustain such a velocity. Since the drag is a quadratic relationship with velocity (and linear with CdA), then there is a cubic relationship between power and velocity (and it's still linear with CdA).

IOW, a doubling of the velocity requires 2³ = 8 times the power to overcome the additional aerodynamic drag.

A doubling of CdA requires a doubling of power to overcome the additional aerodynamic drag. The CdA change can be as a result of changes in size and/or shape.

*

Cd = Coefficient of Drag (dimensionless)

A = Effective Frontal Area (SI units m²)

CdA = Cd x A (SI units m²)

** assuming CdA is constant over the range of Reynolds numbers in consideration for vehicles - which is a reasonable assumption for most motor vehicles and speeds.