Why are transmission equipped with many different forward gears ratios?

Answer 1

An internal combustion engine in a car operates over a fixed speed range, for example 600rpm (idle) to 6000rpm (max power).

If your car had only one forward ratio, say, 1st gear, your car wouldn't go faster than about 25mph before it hit 6000rpm. Although 1st gear would be quite forceful and allow the car to start from a standstill very strongly, the engine would quickly reach its maximum speed before the car was going very fast.

In contrast, if your car had only 5th gear available, the car could be made to cruise at 60mph for example, and the engine could still be within its comfortable speed range. BUT, the car would hardly be able to move off from a standstill without a lot of clutch slipping and wear. The acceleration would also be very sluggish until it reached say 40mph.

By equipping the car with higher gear ratios (which increase wheel speed but reduce torque), the engine speed can be reduced while the speed of the car can be increased. So in 5th gear, your engine does about 3000rpm while the car is speeding along at 60mph or more.

But your car needs more than one gear clearly because there are times you need to start forcefully for climbing hills, or with a lot of acceleration. The lower gears (1,2,3) will allow you to do this. When your car is cruising, the lower gears will turn the engine too fast, therefore to save fuel and wear, the higher gears like 4th or 5th will help turn the engine slower while allowing the car to maintain a higher cruising speed (although the force/torque will be at the minimum).

If our cars had electric motors (or even steam engines), the need for gearboxes would be virtually nill. Both steam and electric power can operate over wide ranging speeds and provide immense torque from 0 rpm - something not possible with traditional internal combustion engines. Therefore things like a clutch and gearbox would be a thing of the past. There would be one ratio (middle ground between 1st and 5th most likely...or probably closer to 5th) and this would be sufficient to power the car over all speed ranges.