Okay, here’s my two cents. I am an ex-truck driver. The states that mandate the left lane is for passing usually say that you can exceed the limit a little (about five mph) to pass someone. This helps to keep the lane clear.
Second, people sometimes slow down when passing a truck because they (a.) are worried when the truck has moved towards the center line, as they should be, since your 4,000 pound car is no match for an 80,000 pound truck, Keeping an 80,000 pound vehicle steady is harder than it looks, given the weather, the wind, road conditions, etc. I’ve seen semi trucks blown completely over by a good stiff wind. (b.) they want to be sure the truck driver sees them, i.e. if you CANNOT see his mirrors, he CANNOT see you. You should really find out where a trucks blind spots are and avoid them, and (c.) they are really cautious. This can be just as much of a hazard as being too reckless.
Semi trucks take (at least) ten times the distance to stop as it does in your car. Your brakes are hydraulic; his are air. Hydraulic brakes are instantaneous when you stomp the pedal – air brakes take a little longer to engage. Giving any truck a wide berth and all the room you can is not only courteous, but it is also might to save your life. If you cut off a trucker and he runs over the top of your car, odds are greatly in his favor that you will die and he will hardly get a scratch. Sure, he’ll lose his license, but that won’t do you any good, you’ll be dead.
I’ve seen what truckers can do when they are pissed at someone with no courtesy on the road. Most truck drivers are professional at all times; but there are always a few bad apples in any profession. You piss one of those nutjobs off, you might just find yourself run off the road right into a very conveniently-placed DEEP ravine. Truck drivers usually know every inch of the roads they drive, as they drive them on a daily or weekly basis. Piss them off at your own peril.