If a car is traveling at 100 mph, what is the speed of the wind coming off the car, is it less or more?

Answer 1

The answer depends on the speed and direction of the prevailing wind, the shape of the car (i.e. drag coefficient), and the point and direction at which the windspeed is measured.
Assuming a calm day (i.e. prevailing windspeed of zero), relative to the car, the windspeed measured from the rear of the car along a line parallel to the car's direction of motion is likely to be slightly less than 100 mph due to the dispersal of the air in directions other than the line parallel to the car's direction of motion.

Answer 2

It's less, since the car is actually pushing the air out of the way, it does cause a breeze. Also you will notice when that car passes you, the sound becomes different, this is called the Doppler Effect.

Answer 3

of course the answer is less