Which has more aerodynamic drag, an auto or a bicycle?

If in free fall together, the bicyle might reach the ground sooner because of its relatively less wind resistance. In a vacuum, they would both reach bottom at the same time, according to the apocryphal tale of Galileo Galilei and the Tower of Pisa. I find it hard to presume that the car would reach a higher speed than the bike. The likely average weight for a U.S. car is about 4,000 pounds. Most bicycles weigh less than 40 pounds. The rolling resistance at the start should be much less for the bicycle.

I would have thought the same but if you were in a regular car going down a grade, you might hit 60 MPH, it is unlikely you’d hit those speeds on a bike.

I don’t see why a bicycle can’t reach speeds greater than sixty mph, if the grade was straight. Besides the ability of the cyclist to control the bike, what would be limiting factors?

I don’t know what the limiting factor is and I also do not have data. I just seem to remember going down hills coasting and passing cyclists who were also coasting. Maybe I am wrong but it sure seems the car would get a higher velocity down the same grade.

My guess would be that the cyclists were applying their rear brakes just enough to prevent going so fast that they could not maintain control of their bikes.