It's probably hard to find a manual trans for that car, might be $1000 in used parts and a fair bit of research to find something that will fit and pedal assembly, etc, to see if it is even feasible.
I don't believe you have a lockup converter (don't really know for sure) so you should be able to leverage a noticeable benefit on the hiway as well as around town.
Of course adding a basket/trailer to your bicycle and using that for local trips is a real good eco bargain/quick fix too as you have observed.
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WINDMILLS DO NOT WORK THAT WAY!!!
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