OK, I know this thread is old, but I just seen it today... So yeah. I also over analyze everything, so come prepared.
Tony I am glad brought this up! The whole governor being constantly active thing was something @OK12V and I were trying to convince the world years ago to little avail.
However, I'm not sure I completely agree with the fuel saving aspect of it.
2 case scenarios:
1st. You're going a down hill and you don't
hit the clutch.
Your engine will be running at the same RPM you were cruising at, say 1800RPM which obviously takes power to spin whether that power is being created by fuel or or drained from your momentum. The power to spin the engine will also be going through the differential, T-case and Transmission, so there will be friction losses there as well.
So at the bottom of this hill, you won't have used any fuel, but you will have lost speed due to the engine drag.
2nd scenario. You're going down a hill and you do
hit the clutch.
Your engine will be spinning at ~800RPM, and there will be very minimal friction loss going through the transmission. You are however burning fuel to maintain your idle RPM.
So at the bottom of this hill, you will have used some fuel, but you won't have lost speed due to engine drag.
So it becomes a balancing act between fuel used, and momentum lost. It would be interesting to do testing on this, measuring fuel usage on a hot engine, and then seeing how much speed you lose with and without clutching. This would be an entertaining experiment, However simply using the Law of Conservation of Energy, we can come to a basic conclusion.
In scenario #1, the engine is spinning faster thus creating more friction, and so is the drivetrain. So by not
hitting the clutch, more energy gets wasted to friction and thus heat. Now there's the argument of the efficiency that the fuel is being burned at, and whether that makes up for the extra friction. However we must remember that this is the same fuel that must be burned to make up for speed lost while coasting, making it somewhat of a null-argument.
Now, if this hill is rather steep, so riding the brakes is required, or if you'll be braking shortly after the hill for a turn or stop sign, then the speed lost while coasting down the hill has no effect on the overall fuel usage of your trip and it becomes a no-brainer to coast the engine without hitting the clutch, might as well slap the exhaust brake on as well.
We are also truck people, so it's worth mentioning that if you're hauling a heavy load up this hill, than crest the top and are going back down. If you hit the clutch letting the engine return to idle, this CAN cause the engine to overheat due to your cooling system performance being directly proportional to engine RPM.
I guess the proof is in the pudding, so do I clutch down every hill??? Heck No lol. I'm not a MPG junky, nor do I believe that the extra hassle or clutch wear is worth the cost of fuel.
Now, if you've made it this far, you're probably wondering where all this crap came from, how can this poor soul devote so much thought into such a pointless topic. The answer is simple. You try driving around in circles all day on a tractor going 8 mph. The mind wanders. And this tractor has a mechanical fuel shut-off, which makes playing around/testing really easy.
Back to the governor involvement, Tony's test makes it pretty clear that it decreases fueling when the RPM is over the RPM set by the throttle pedal, even to the point of cutting off fueling completely. Here's a vidier showing that the opposite is true as well, that the engine will increase fueling if the RPM drops below the RPM set by the throttle pedal.