Originally Posted by wphamilton

If I can dig it up I could just post it here.

Re vacuum, it's not very pertinent to coasting down a hill or biking to work. That's sufficiently handled with rho in the drag equations, which you utilize for terminal velocity. I think for our purposes we can consider the air density to be constant during a ride, and even disregard the actual value entirely if you're just looking for a ratio of drag coefficients (it divides out).