Ret, sorry for using your idea, I just thought that if I have few minutes free I can check this :P
I took 50 lauan wood to make Big Wooden Gears. 1 wooden gear is 4 minutes, 50 is 200 minutes. Any time-cutting furniture should cut full minutes. An here are the results:
TIme of manufacture:
Base - 200 minutes - 100% - 0% time cut
With Windmill - 194 minutes - 97%% - 3% time cut
With Stand Bell - 194 minutes - 97% - 3% time cut
With Wooden Bell - 196 minutes - 98% - 2% time cut
With Air Conditioning - 170 minutes - 85% - 15% time cut!
With all - 154 minutes - 77% - 23% time cut!

This all adds up:
3%+3%+2%+15% of alone cuts = 23% of cut with all equipment. So there are no "combos" inluded. It's just the fact that Air Conditioning really shortens more than it should :)