That's true, I was overlooking that. A stiff headwind could produce the same effect as a higher speed...thanks!

Even stiff crosswinds getting under the car will mess up the FE. I've noticed a lot more stability in crosswinds since installing the side fairings.:D

that depends on the setup. all my mods are for aero and none are for rolling resistance. i have a feeling that the cross over for my car is much higher. the same is probably true for the aerodynamic sports cars with really fat tires.

Aerodynamic drag begins to become the dominant force to overcome on most cars between 30 mph and 40 mph.

For around town stop and go driving, liberal use of properly-executed aeromods might only improve fuel economy 1-5%. In highway driving, the effect is more profound, around 15-40%. Of course, it all depends on the car.

I'd suggest taking a look at the link I suggested earlier in another thread. It's an excellent site that allows you to see what the power requirements will be at the wheels for a particular bodyshape, weight and set of tyres. After that it's up to the efficiency of the engine to produce that power.

Here are two graphs I made for a hypothetical camry sized car, keeping the same set of tyres on it. I suggest having a play, alternately setting the different coefficients to zero to see the raw effect of the other on the power required at the wheels.

https://i3.tinypic.com/167s7px.jpg