understanding 3 speed gearbox conversions

[Bob Culver]

I am intrigued by the comment that 3 and 4 speed cars do not mix well on runs, because for stock non sports rpm in 3rd of 4 and 2nd of 3  are about the same. The 3 speed rev lower in top but are mostly much lighter. I dunno to what extent the crash boxes hamper change speed but I usually double declutched downward on the 4 speed syn anyway and lost neglegible road speed when pushed. Upward changes are certainly often a handicap on large old cars as the delay can be very long. The local owner of Sports 20 claims Sevens often do better on hills as his car once committed cannot change up until the road distinctly flattens.
The near doubling of rpm 3 to 2 of small 3 speed cars is not uncommon. Ford Y, Ten etc.