Say for example a 125cc motorcycle with 6 gears has a top speed of about 75mph, why can’t they make it with 7 gears and surely that’d increase the top speed?
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When talking about gears, always remember this:
Transmission gears trade speed for torque (and vice versa).
Torque is force x distance, when applied in rotation. So you put a wrench on a bolt, the torque applied to the bolt is the force you apply to the end of the wrench multiplied by the length of the wrench.
Your engine has a fixed amount of power it can deliver across a range of engine RPM. The result of the power is high rotational speed, but low torque. Your wheels need a lot more torque to start turning to move the car, so your lower gears trade the rotational speed of the engine for more torque. As the car builds momentum and inertia, less torque is needed at the wheels, and more speed is required, so you shift up.
If wind resistance, and rotating friction were not concerns, you could just keep adding gears and get more speed, but they (& other losses) exist, so even at the top end, you have to have a minimum amount of torque at the wheels, so you can’t keep adding gears (& you start running out of room in the transmission case).
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