I also checked into varying fan speed for my M1MC090BW. The house came with a 4 ton ready coil cabinet. The outside unit is a 3 ton Miller with a matching A coil inside with a "flowrater" orifice setup. You cannot get this metering device with a modern refrigerant, it has to come with a TXV one. This all has to do with SEER rating, which typically is better than than the one I have. I read the Interthem material which recommended a certain indoor fan speed for the particular setup I have, and I set it that way. Yes, it's a matter of hooking up the designated wire for corresponding speed. However, from my experience even with window shakers, install "reversible" ceiling fans wherever possible, they work awesome with cooling (and heating) They seem to help keep the temperature "even". It's a little trick to install them properly, don't bolt them up to a plastic box! They have to be mounted in the ceiling securely, sometimes with a crossbar fixture you have to shove up in the fixture hole.