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When you drive your manual transmission car, you have to select what gear your transmission is in. What this means is that you're telling the transmission how, exactly, it's supposed to channel the power your engine is generating to your drive wheels. This works the same whether you've got a front-wheel, rear-wheel, all-wheel, or four-wheel drive car, although the logistics may differ by model and/or manufacturer.
The gears in your transmission are similar to the sprockets on a 10-speed bicycle, only they're enclosed, and they directly engage with each other --- there's no chain connecting them across a vast expanse of space. To better understand how your manual transmission works, take your 10-speed bicycle for a ride, and pay attention to what happens when you change gears. Lower gears are good for getting your bike going at lower speeds, and provide more torque to your wheels to get them going from a full stop. Higher gears are better for coasting once you've reached a higher speed. Your legs will notice right away that you're expending less energy to maintain your momentum.
Now that you've had the experience of being a forward-moving engine, you might understand a bit better how your manual transmission and engine work together. Lower speeds require higher torque to get going, just as with your bicycle. By contrast, at higher speeds, less energy is required --- and you can directly control how much stress you're putting on your engine by shifting it into a higher gear at higher speeds. Notice how your RPM drops when you do this? That's why (see Reference 1). -
One big difference between your bicycle and your manual transmission car is that your bike doesn't have a clutch. The clutch eases the shifting between gears. Notice the chunking noise and sensation that happens when you change gears on your 10-speed bicycle? The clutch disallows that from happening when you're driving your manual transmission car --- as long as you use it correctly.
Different engines have different "sweet spots," at which you're getting the most bang for the least energy expenditure. Driving your car in the most efficient way possible will require you to pay attention with all of your senses. Listen to the engine --- does it sound like it's struggling? You're in too high a gear for the speed that you're going. Look at your tachometer; it'll confirm what you're hearing. Engage your clutch, lightly tap the throttle, and downshift to a lower gear --- then listen to your engine purr happily. Are your revs too high, perhaps even approaching redline? Engage your clutch, upshift, and ease onto the throttle to bring that needle back down into a less stressed range for your engine.
When engaging your clutch, do so smoothly. Ease on and off until you get to know the characteristics of your car. As a general rule of thumb, always engage your clutch whenever you're shifting gears --- whether it's downshifting or upshifting. Don't keep the clutch engaged longer than you have to, or you'll wear it out. At first, you may not know how long you have to keep the clutch engaged while shifting, and that's OK --- with experience, you'll learn what your car prefers, and you'll be a better driver for it.
