That probably indicates they took a shortcut and used general-purpose routines for signed numbers that were lying around instead of writing more correct versions for positive numbers from scratch. That has no meaning in this context - all numbers should be treated as positive numbers. When the numbers get extremely large, the magnitude bits overflow into what is sometimes considered the sign bit. What is does NOT allow you to do is any actual reduction in minimum fan speeds, regardless of whether it allows you to set lower numbers. They will spin faster at idle, and it is possible they may move up to even higher speeds more slowly. What SMC Fan Control and similar software allows you to do is put a new Floor under the fan speeds. The power supply fans were designed by a different team, and may not have the same controls. If the diagnostic indicates a MOT error, that is likely to be a Fan Motor that is not showing high RPMs at High speed settings, and may be defective. One possible exception: Most of the fans do have an RPM output indicator, which tells whether they are spinning properly. Fixing the temperature inputs is where you should place your energy. If you are getting racing fans, that is probably based on Bad Temperature inputs. They make a feedback loop based on measured temperatures. The fans in the Mac Pro are controlled by Software in the System Management Controller. I think you are 'barking up the wrong tree'. I'd like to make sure my settings are fairly optimal before I do anything intensive. I've been trying to find a diagram showing which fans cool which parts of the system, but I mostly get results about fan noise, or minimalist diagrams that don't tell me all I want to know. The fan names give basic descriptions, but not a lot of detail beyond that. This works really well for lowering the fan speed (and thus noise), but I understand that different fans cool parts of the system besides just the CPU. What I've ended up doing so far is download Macs Fan Control and setting it to base the fan speeds off CPU temps that don't give faulty readings. Yes, I've already tried resetting the SMC and cleaning it out - nothing.) Sometimes, it will even show negative temperatures, in which case the fans will really go into overdrive. (Using iStat, it will sometimes show unreasonably high temps on a couple cores immediately after turning it on, whether or not it's been off for awhile. In my case, I can get really high fan speeds, and the problem seems to be due to a faulty temperature sensor. I'm one of the many people who have problems with the fans.

So I have a 2008, 2.8 GHz 8-core Mac Pro tower.

Apple mac pro tower