OP here - You Nailed It, Big Aura! And also provided data confirming the relative greater decrease of running compared to cycling.

My question was prompted by youngsters blasting by me when I ride. How does my 15 mph compare to his 20 mph? (I realize I am revealing my shallowness! Sobeit.)

This is my extraction from the chart:

For anyone curious about the last column, I had already computed this (speed in cell above * [1.0 - change]) before I realized I could easily compute their actual speed. I left my arbitrary column in because my original point of reference was how 20 MPH drops off. Maybe others do also. FWIW, I can change "20.0" to any number.
(Sorry I don't know how to have the image show in-line)