How RPM works on a manual transmission


I understand the basics of how the clutch/gears work and that if the input speed is too high for the gear, you shift to the gear that can handle the higher speed. But what makes that gear able to withstand that? Isn’t the gear smaller, which would make it spin faster?

Hope this makes sense to car people.

In: Engineering

No clue what you mean by input speed.

But If you’ve ever ridden a bicycle it’s similar.

A smaller gear means that the engine (you) have to pedal less to spin the tires one rotation. so for a given engine rpm a higher (smaller) gear means you go faster (more rotations of the tire)

It’s actually easier to understand by looking at a bicycle, which is the same kind of system but more linear. Imagine that the biker always wants to be pedaling at a constant rate.

A smaller (higher) only requires a few teeth (fewer crank rotations) to turn the wheel once. However, since it’s trying to spin so much mass, it requires a lot of input force to overcome the inertia of the mass. If the wheel already has a lot of rotational inertia then a lot of torque isn’t *needed* to maintain a high speed.

A bigger (lower) gear requires many teeth (and many crank rotations) to turn the wheel. However, the size of the gear gives it much more torque, so it gets the wheel up to speed with less effort.

And since the biker/car always wants to be rotating the input (cranks/engine) at a constant rate, they must shift to higher and lower torque gears depending on the needs.

As for how the gear can *physically* withstand it – it’s a thick chuck of metal that really only has to spin (transmit rotation). As long as the system’s not getting locked up on anything it is plenty strong enough to transmit rotation.

Hope that helps.

Think about it like a geared bicycle.

What matters is the interaction between *two* differently sized gears. This is called the gear ratio.

The first gear on a bike is smaller than the pedal gear, so the pedal gear rotates more times than the drive axle.

As the gears grow larger, the ratio changes so that the pedal gear – or motor’s output shaft – turns the drive axle more than once. The further away from 1:1, the more energy it will take to turn the drive axle, since the output shaft is rotating it more times with the same amount of revolutions.

To answer your question, the gears use the momentum of the flywheel, the clutch, and synchros to match the rotation of the trans input shaft with the rotation of the next gear in sequence. The gears are connected to the input shaft through shift linkages.

At the top of the first gear on a bike, you are pedaling very fast. Shift up to second, you are pedaling slower again. The change in gear ratios allows that energy to be maintained but with a lower RPM. Because the gear got larger, I have to put more energy into turning it. This is why when you shift into a higher gear, RPM go down.

It works exactly the opposite when downshifting. Think of going down a hill on a bike. I am in 6th gear, pedaling at a mild pace. I shift down one gear, I am pedaling like a madman. One more gear down, I blow my knee out. This is because the input energy stayed the same (or increased), but the force required to turn the axle decreased. On a bike the knee blows out, on a car this will destroy the engine.

To avoid this, you want to let the RPM go down so that when you downshift the RPM doesn’t exceed the redline. Preferably nowhere near the redline. If you aren’t a fan of downshifting, just throw it in neutral and apply brake! Just like an automatic.

Hope that answers your question!

Because your transmission can only operate within the perimeters of what an engine can do.

If an engine can only turn at 1,000 rpm. It can only cycle 1000 times in a minute. the moving parts can only do so much work in those 1,000 rotations. You use your transmission to either lengthen the distance/time of the work to be done (taller gears) or you shorten the distance (smaller gear) but make the work “harder”


The glory of modern transmissions is we have multiple gears.

The REALLY large drive gears and really short output gears. This means the bigger gear has to move a bigger distance, over a longer period of time to achieve the same work as the shorter gear. But this work is accomplished easier. The problem is, that big gear takes a lot of RPM’s to do one full cycle. So in a low gear, you’d have a lot more “torque” or power, but a lower top speed. since it takes longer to turn that big gear.


so you shift into 2nd, now you have slightly less torque, because the drive gear is a touch smaller, and the output gear is a touch bigger. You give up torque and gain a higher top speed.


Then you eventually get to your final gear, you have a lot less torque, but a much higher top speed. This is why you’ll probably stall your car if you try moving from a stop outside of a low gear like 1st or 2nd. To get moving, your engine needs the gear advantage. But once you start moving, and you’ve ramped up some speed, it’s easier to maintain a speed than it is to get there. so you can give up some of the torque once you’re moving.


Source: I’m a motorsports tech who specializes in Porsches.