If you are driving a car with a manual transmission, you will have to shift gears frequently, but there are also times when you have to shift gears manually when using the automatic transmission. Let's deal with the manual gearbox first.
When to shift gears with a manual transmission
All modern cars with a manual transmission have at least 5 forward gears (usually 6) and one reverse gear. In addition to the obvious reverse gear shift scenario, when you want to go backwards rather than forward, there are several scenarios where you will need to shift gears.
As speed increases, you will have to shift to another gear to keep the engine in the optimal RPM range for a smooth and fuel-efficient ride. Under normal driving conditions, this means a shift upwards if the tachometer reaches 2500-3000 rpm. With hard acceleration, you will change gear as you feel the acceleration slowing down, which typically exceeds 6000 rpm for a gasoline engine, but less for a diesel engine. Normal cruising RPM range will be 1800-2500 RPM.
When braking, you will have to downshift before the RPM drops below 1000 RPM, otherwise you will have no power if you need to accelerate; the car will eventually stall if the revs become too low. Ideally, you should keep your vehicle in gear at around 2000 rpm unless you are stopping.
You can also downshift to use the motor as a brake. Truck drivers do this all the time. Shift down one or two gears on long downhill runs. You don't want to reduce too much - the car shouldn't need any accelerator pedal to maintain speed, but it will need much less pressure on the brake pedal, which will prevent the brakes from overheating, which can cause the brakes to drop out. It doesn't matter if your engine sounds like it's racing a bit - you can let the tachometer climb but don't let it go red.
You should downshift if you're preparing to overtake. This ensures that you have available power when you decide to move, rather than seeing the opportunity and having to shift gears.
Of course, driving in a low gear for a long time will result in additional fuel consumption.
If you park the hood down, leave the car in reverse; if parking facing upwards, leave the car first. If your handbrake fails, gravity will have a hard time getting your car into the gearbox in the opposite direction. However, be sure to put it in neutral before starting the engine.
If your manual car has cruise control, gear shifting or often just "plunging" (pressing) the clutch is enough to turn off the cruise control. Of course, you can always turn off the cruise control with the included button or lever
When to shift gears with an automatic transmission
Simple automatic transmissions often have L, 2, D, N, R and P. These types of automatic transmissions do not provide much engine braking. In fact, they will just keep shifting gears until they run out of gears. Therefore, on steep descents, it's a good idea to shift to 2 or L to force engine braking.
For overtaking, automatic transmissions usually downshift in response to your foot setting, but you can anticipate this by shifting gear first.
Modern automatic transmissions have up to 9 gears and can often be "semi-automatic", in which the driver can select gears using the gear lever or paddles behind the steering wheel. This makes it much easier to choose when and how many gears to shift. The vehicle is in D or S mode (which changes after shifting gears).
The default mode for an automatic car is economical driving, which means that it will usually use the highest practical gear. But for advanced riders who drive on winding roads with a lot of hill changes, the control of the gearbox allows for a much smoother ride. Experienced drivers move downhill just before entering the ramp to avoid overloading the engine. They can also shift gear before cornering when braking to avoid mid-corner gear shifting which can make the car alarming.
Finally, automatic transmissions can "hunt" between gears under certain circumstances. This can happen on less powerful vehicles when they are tight on a steep slope. The gearbox shrinks down to deliver enough power, but when the tachometer reaches a certain level, it goes up. If the hill is steep enough, the motor does not have enough power, so it descends again and the cycle repeats. Holding down the downshift manually prevents this hunting.
When should I change gear?
- Your speed becomes so high that your revs exceed 3000 rpm during normal driving
- You accelerate hard and accelerate slows down (note: do not let the tachometer needle enter the red line)
- You slow down and the revs drop below 1000 RPM
- You expect to need more acceleration, e.g. approaching a steep hill, preparing to overtake, etc.
- You want to keep the brakes by using engine braking
- You want to cancel cruise control
- You want to ride smoother where the road has many curves or changes in elevation.