Observational data: I routinely pass single upright bikes going downhill, both on a tandem with my wife/stoker, and on my recumbent. I weigh a bit less than 200#. Stoker weighs a lot less. The bikes are a bit heavier than average. Moderate cross-section tires running at about 90-95psi on both bikes.

The speeds are directly affected by my/our energy input and by physical forces beyond our control.