kudos for noticing the difference in RPMs in morn and after warm up. Some people don’t pay no mind.
These posts are touching on why you would have an RPM guage. Some tow trailers and such and need to monitor RPMs, especially when out of overdrive.
Now. here’s why your RPMs are different in am and after warm up. There is a computer program that is called P.R.O.M. and no it’s not the high school party you never got asked to. 
it stands for Program Read Only Memory which stores all the vital information to help the vehicle run, especially in cold weather. That’s why sometimes a fuel injected vehicle tends to start up quicker than a carbuerated vehicle. Now that the car is running, yes, the idle is about 1000 to 1200 RPMs, but after a while of running, now the exhaust is warming up. Hear of an 02 Sensor? Here’s where it comes into play. When it reaches a certain temperature, it sends a small electric current to the computer which opens the circuitry (opens or closes, i get it confused). anyway. this is telling the computer to take over because everything is warmed up. I live 4 blocks from a main county road. I drive 35 miles to work. I don’t usually let it warm up unless its freezing. Onb the highway, running 60 mph @ about 2300 RPMs. Then my temp., guage comes up and i can see the RPMS drop to 2000 even.
Good post and observation.
JP#3
Open circuit: When you floor it or warming up a stone cold engine as described. X amount of air comes in, Y amount of fuel injected whether or not all the fuel is burnt.
Close Circuit: Use the O2 sensor to measure the amount of left over O2 and “feedback” the information to the injector. The injector tries to get it right the next time and the O2 relays the results back to the injector. Info travels in a loop. That’s why its feed back.
Indy, most tachs have a yellow and then red zone. This is pretty much straight forward. Like airspeed on an airplane, this is the don’t exceed zone.
Most vehicles powerband is in the 3-4000 rpm range. The engine is rotating[revolution] at this speed [per minute] Thus “RPM”.
To get optimum gas mileage ,the tach should be between 1600 and 2000 rpm. However, this range is low if you are climbing a hill or need instant power. This is a good RPM on a flat road ,cruising situation.
Alot of manufacturers try to keep the engine in the 2-2500 rpm range at cruising speed so the engine is not “lugging” under pulling load.
Idle speed should be around 8-900 rpm at idle.Fluctuation at idle could indicate there is something wrong with the running of the motor. Very minute fluctuations should not be of a concern, as the computer is always adjusting idle sped .Idle speed in “park” is usually 1-1100 rpm.
Sometimes poor quality fuel will cause minor fluctuations.If the motor is fluctuating two or three hundred(not thousands) of RPM,the motor should be checked out. (possibly a tune-up related issue)
The engine will turn higher RPM on start-up to warm up the engine and burn off excessive hyrocarbons produced by the initial higher fuel delivery on start-up.
I worked 31 years in a high-tech electronics factory. One thing I learned over those years is to just look at something you don’t fully understand. Don’t wreck of course, but start noticing exactly what happens to the tachometer (when traffic is such that you can glance quickly at it without wrecking.) It might be a while, but eventually you will start to make sense out of it.
That is how some of our smartest contributors, men such as Docnick and OK and others learned so much. They were not born knowing everything. It takes observation and thinking about it.
When you start the engine, the rpm will usually go up to perhaps 700 rpm or so, and will settle smoothly at that number. If it bounces around, you may have an idle speed actuator problem.
When you start moving, it will perhaps be somewhat different on automatic and manual. But, when you press the throttle, it will go up as the engine runs faster. When you are in gear, it will increase as your speed goes up, then when you shift gears, it will drop back again, then increase again.
When you are at a stable speed, the rpm will be stable also in most cases. You will need to learn what is correct for your vehicle. If it varies, did you shift down on a hill, or have to give it more throttle to maintain speed against the wind, or going up hill, or is there a problem?
As I said, it takes time, and simply observing and thinking is one of the best ways to learn things. As well as keeping your brain working.