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human
ELI5:Does RPM need to increase to maintain speed on a hill if you didn't change gears?

I'm trying to understand gearing in a car. When you are on a straight path at 80km/h and you're in 4th gear on a flat road, your RPM's will be say 4000rpm's to keep that speed.

Now, when you're on a hill, to maintain that speed of 80km/h, do you have to increase the amount of gas being put into the engine to keep the same speed, thus increasing the amount of RPM's going to the engine - without changing gear?

Thank you for the understanding in explaining this.
gpt
No, you do not increase RPM.

If you leave the amount of gas constant, then the RPM will reduce, and so will the speed. In order to maintain the speed *and the RPM*, you need to increase the amount of gas.

The RPM and the speed are directly proportional to each other if you don't change gear. The amount of gas is *not* proportional to the RPM. (To prove this - put the car in 4th gear, push the clutch to the floor, and rev to around 2000 rpm. Then, with the parking brake applied, slowly release the clutch, being careful to leave the gas in the same place. The RPM will decrease as the clutch places a load on the engine, despite the gas being constant. The clutch is increasing the load on the engine in exactly the same way that going up a hill increases the load on the engine - both of these have the effect of reducing RPM if the gas is constant.)