>>1851287you need to think about how a gearbox works man, these are very like 101 questions that i hope anyone who drives a car would inherently understand.
Consider a manual car (with no torque converter). The shaft driving the wheels is directly connected thru a gearbox to the engine.
If my engine is revving at X rpm (e.g. 2000) and my gearbox has a ratio of Y (e.g. 1:20) then my wheel rpm is X*Y (e.g. 100). My wheel rpm * circumference = the speed of the car.
If i then shift down a gear, i am still travelling at the same road speed. So what happens to the engine? In an old car without something called a synchromesh, you would just crunch the gears because the gears connected to your wheels are spinning faster than the gear you're trying to engage. In those days you had to let the car slow down to bring yourself into the lower gear without damaging the gears. Modern manual cars have a system called a synchromesh which gives you more room to breathe here. Nowadays when you shift down there's an intermediate cone engagement that uses a hydraulic coupling to work a bit like a torque converter and it will bring your engine up or down in RPMs to match your output gear speed, within a certain range. It can't handle crazy abuse but it gives you much more room to breathe.
The purpose of a gearbox is to allow you to go at lots of different speeds with an engine that only wants to spin within a small RPM range. If an engine is spinning too slow it stalls and stops. If it spins too fast it will be damaged (this is the "red line" on your RPM meter in your car, and depends on the engine). For each engine, there is an ideal rpm for maximum power and a different ideal rpm for maximum efficiency, which changes with different fuels, temperatures and pressures. The purpose of a gearbox is to make it so if you want to go at a certain speed, you can run your engine through the gearbox in a particular gear that means the engine is happy to give you power at that speed.
