Charles Talleyrand wrote:
Alternatively, there must be planes that have used both fixed and
constant-speed props. Anyone know the difference?

Don't know about the Cessna 150, but I'm pretty sure some owners of
experimentals (such as the RV series) have tried both fixed and later
constant speed props on the same engine and airframe. If interested, I
suspect one could find numbers mentioned on the forums on this web site:
http://www.vansairforce.net/