My brain cant seem to grasp the concept behind this sorry
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Engine RPM * Gear Ratio = Wheel RPM
That’s how it works. When we say a “low” gear, the number for the gear ratio is low. A “high” gear has higher number for the ratio. Most people don’t know their exact ratios, but it doesn’t matter. We don’t know wheel speed in RPM either. What matters is the relationship, and how the multiplication works.
If the gear ratio goes down, but wheel speed remains the same, then engine speed must go up to compensate and keep the equation equal. If you don’t, something gives… the engine speeds up and/or the car jerks to slow down.
They don’t need to be higher unless you want to go the same speed in a gear lower as before. The “gear” is just the ratio of your engine’s shaft spinning rate versus your wheel spinning rate. The higher the gear, the closer they are to 1:1, the lower the gear, the more force the engine can exert for a given RPM in exchange for speed.
It works the same way as riding a bike. When you need to go up a hill, you switch the chain to low gear, and you pedal faster to go the same speed but it’s easier on you. The gear ratio is low, because it takes more turns with your feet (numerator) to complete a full turn of the back wheel (denominator) than it would if you were in high gear.
A smaller gear needs to rotate further to cover the same distance (on its outside edge) than a larger gear does. So in order to make the wheels of your car rotate at the same speed, a lower gear needs to make more rotations per minute, compared to a higher gear.
It is an identical process to riding a multi-gear bicycle if you’ve done that — at lower gears it’s much easier to pedal, but you need to pedal much faster to compensate. At higher gears you have to push harder, but each cycle of the pedals gets you more speed.
Low gear means fewer rotations of the wheels for a rotation of the engine, which also means more rotations of the engine for a rotation of the wheels.
High gear means more rotations of the wheels for a rotation of the engine, which also means fewer rotations on the engine for a rotation of the wheels.
Rotation of the wheels is a multiple of your traveling speed because the wheels are rolling with the road.
The tachometer (thing showing rpm) is measuring rotations of the engine, so at a set speed (set wheel rotations) the numbers (rotations of the engine) will be higher in low gear.
They don’t “need to be” higher.
The engine can ‘spin’ from a very low RPM of about 800 rpm to about 3000 rpm. Below 800 rpm the engine can stall (shut off), the engine struggles very hard to go above 3000 rpm.
The gear you select engages the spinning engine to make the wheels spin. At the lowest gear you can go from not moving to some low speed, let say 30 MPH. Your engine is working as hard as it can, 3000 rpm, to go 30 MPH. To go any faster, you have to switch to the next higher gear which goes from 25 MPH at 800 rpm to (let’s say) 50 mph at 3000 rpm. To go any faster you have to switch to the next higher gear. Etc.
It’s the same as a bicycle. In low gear you just can’t spin your legs any faster at some point. In high gear you can’t push hard enough to get started from not moving.
(note: the actual rpms and MPH in my example are just made up. When I drove a manual stick I just went by the sound of the engine, not the rpm)
If you ever want to know exactly what’s happening with the RPM of an engine, the gear ratio and the RPM of the wheels, just try riding a bicycle with gears. Your legs are the engine. You’ll see how much harder it is to work against a higher gear when you’re going slow, and how you have to pedal very fast in lower gears to keep your speed up without shifting to a higher gear.
The tachometer, which is what displays RPM’s, is showing you the engine speed. Engines have a fairly limited range of speed they can operatate so a transmission is used to convert the narrow range of engine speeds to a wide range of wheel speeds. This is done by have several different sets of gears that have different gear ratios. Gears are called low or high because of the vehicle speed they are intended for not because of the engine speed.
A gear ratio is input speed/output speed. First gear in a car may have a ratio of approximately 5:1, which means for every five revolutions of the input gear (connected to the engine), the output gear (connected to the wheels) rotates once. However, torque is inversely proportional to speed, meaning the output while turning slower will be able to apply more force to accelerate the vehicle
When you want to accelerate you need to apply more force to the road via the tires so you want more torque so a high gear ratio is used to convert high engine speed into lower wheel speed with higher torque. As you shift to higher gears the gear ratio decreases, so the torque available to accelerate you decreases but the vehicle can travel faster without over spinning the engine. As you reach cruising speed you need a lot less torque because now you just need to overcome air resistance and friction to maintain speed so cars can shift to a higher gear with an even lower gear ratio.
RPM is like the number of steps you are taking. When you first start to run, your steps will be shorter and more frequent compared to when you get going–especially if you’re going downhill and can switch to a “high gear” and take longer, less frequent steps.
2 different machines. your engine and the transmission. rpm is engine speed, gear is the transmission ratio preset. first gear is the highest ratio for engine speed vs driveshaft speed. giving completely fake numbers here: at 1500rpm engine speed and first gear being 10:1, the driveshaft will be spinning at 150rpm. if 1mph=10rpm of the driveshaft, then at 1500rpm engine speed in 1st gear, you’ll be traveling 15mph.
at the same 1500rpm engine speed, if second gear is 5:1, the driveshaft will be spinning at 300rpm. thus, you’ll be going 30mph. to reach 30mph in first gear, you’d need an engine speed of 3000rpm.
if 3rd gear is 2.5:1, at 1500rpm engine speed you’d achieve 60mph. many engines are simply not capable of spinning faster than 6000rpm, and wont last long at that speed due to heat. not to mention how fast you’ll burn fuel.
by having many gears, you can keep the engine speed low while moving quickly. this extends the life of the engine and reduces the cost of traveling fast.
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