[B_part]

Interesting question: when shift and why? Here's my 2 cents:

As someone pointed out, it depends on the car and the gear ratios, but every car has an optimal torque power at a certain rpm. You must know your car to know exactly where it is (in mine it's about 2500 rpm), but the trick is that if the engine is too "noisy" (and only your ear can tell you that), it's time to go one gear up. If the car doesn't respond when you press the pedal, it's time to go down.

[Simplicistic explanation of how gears work starts: skip it if you get bored, but be sure to read at the end, there are tips from my experience]

If you have a bike with gears, you are going to easily understand what I am about to say. If you don't, well, it's gonna be a bit more difficult.
Anyway, changing gears means changing the number of revolutions your wheels do per engine cycle. upper gears mean more turns, lower gears less turns. Since every engine cycle can give you only a finite amount of power, upshifting means you will put less power in a single wheel turn, but your wheels will turn more (=you go faster). [This is not completely true, but it's true enough]

This explains why you can start moving only with 1st or at most 2nd gear: upper gears cannot put enough power into the first wheel turns, and the engine stalls because it cannot turn anymore.
Also, if you start to go fast, you will need to upshift: going at 36 kph means moving at 10 meters/sec. No matter what, if your wheels have a circonference of 1 m, they will need to turn 10 times per second. suppose your 1st gear has a ratio of engine cycles/wheel turns of 50/1 and your 2nd gear of 25/1 (I'm making numbers up). at 30 kph, in 1st gear, your engine will be at 3000 rpm (500 rpsec), in 2nd at 1500 rpm. (Different noise, different fuel consumption, but also different torque).

[End of explanation]

But in practice what should you do? experience will come, just bear in mind that you should downshift every time you want more power, and the gear you are currently using gives you an rpm under the optimum.
And also:

1)Before passing a car, downshift one: that will give you more torque and allow you to accelerate faster, thus completing the manouver faster.
But be sure to watch your speed: if you are already fast enough, keep your current gear - downshifting would not give you more torque.
Example from real life: Fiat Punto 1.2 16V
I'm at 90 kph, 5th gear 2000 rpm. Optimal torque at 2500 rpm. Downshift to 4th gear 3000 rpm, floor the pedal, upshift again at 110 / 120 kph.
I'm at 100 kph, 5th gear 2500 rpm. Optimal torque, just push and go.

2)When you are braking. Downshifting one or more will ease your brake - the engine resists the increase in rpm, thus acting as a brake.

3)Before a turn.
When turning, you need to slightly accelerate - no, I'm not joking, this helps a lot to control the car. Braking during a turn is dangerous - you might lose control.
So downshift one: this way you can benefit from the engine-brake effect that will help you slow down. Also, having more power during the turn will allow you to better control the car.

4)You are cruising along at constant speed. try to keep the gear that gives you optimal performance or slightly under that. this way you'll spare fuel.

[ 01-10-2004, 06:48 PM: Message edited by: B_part ]