[Otto]

Also, you could cut one coil off the spring*, if the spring is cylindrical cross section and has the same pitch such that it will still properly seat on its perch. This would lower the car an inch or more, stiffen the spring rate, but aerodynamically put the wheel/tire further up into the wheel well where it has less exposed frontal area to catch the wind.

Further, as the tire acts as a crude turbine, it throws off 6 vortices, the worst of which is at the upper edge and goes directly against the airflow past the car. You can see this on other cars when driving in the rain on the freeway--the mist turbulence is visible at the upper forward crescent of the wheel well, and can effectively increase the frontal area of the car while buggering the slipstream much more than necessary.

With the wheel/tire further up into the wheel well, less of that upper vortex goes out against the slipstream, as the fender acts as more of a barrier.

Also, the lowered car nose lets less air go under, so less air under means less drag from all that stuff like mufflers, etc..

So, less wheel/tire frontal area + less opposing upper vortex + less under car airflow = less drag.


*This is endlessly debated on the BMW forums, some saying never to do this, others who've actually done it reporting no ill effects if only 1-1.5 coils are cut such that the spring still seats well on its perch, typically the bottom perch. More than that an the spring may dislodge from its perch when unloaded, or you'll have trouble hitting curbs, etc. in the parking lot.