It's been a while since I had an analog museum piece, and even longer since I drove one in iceball country, but I have to think it is related to the throttle linkage. The control pressure cable, connected to the throttle linkage, is what determines the shift point. On my 201 and 124 I don't recall any cold-start mechanism that affected the linkage, but on yours maybe there is? Is there anything that performs a choke function when cold that pushes on the linkage, and is supposed to release when cold? Having an open throttle plate will lower your vacuum too, giving the poor economy reading.

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Prost!