You will need to read the car manual to find out how and when to engage and maintain the lower gears.

There is either a S or L on the gear shif. (Aside from P, R , N and D - that is Parking, Reverse, Neutral and Drive.)

This S or L will prevent the car going into the highest gear. This is the preferred driving mode when going uphill and downhill as lower gears give the car more power as well as higher engine braking.

In some cars, there is also the O/D Off button on the gear stick. This "overdrive off" button is off in default mode. Meaning the car will go into overdrive mode for better fuel efficiency.

When you pressed the button and the O/D Off is lighted on the dashboard, it means overdrive is off and it will not go into the highest gear. This maintain the car in lower gear and maintain power when going uphill. And provide more engine braking when going downhill.

It's called the O/D OFF button. Press the button, it will cut off overdrive. Normal driving, you want overdrive for better fuel efficiency.

From D to L, this you need to refer to the car manual. Some cars got L1 and L2 - you must slow down and not at high speed (already at top gear) when going into L1 or L2 and back to D.

You usually go into L at the foothills, not like changing it later midway. (Or at the top before starting the journey downhill.)

Along flooded roads, you slow down and go into L just before the flooded stretch, not midway.

The O/D Off is a bit different. This can be done at higher speed. A good road example is karak highway, a few kilometers before and after the tunnels, you press the O/D off and maintain constant power (and speed) on the long uphill stretch. Turn it off by pressing the button again after a few km after the tunnels.

L restrict it to 2nd or 3rd gear. O/D Off restrict it from going into 5th gear.

The O/D Off switch will prevent the gear from going into 5th gear. If cruising on highway, this of course will be more fuel consumption.

What car are you driving? Not all cars have this O/D Off switch. Do read my previous post again, as it tells the different btw L (or L1 and L2) and this O/D Off switch.

If talking about Getting Highlands, normally I will switch to L at the top and switch back to D at the polis pondok. If driving Xtrail 2.5, using the O/D Off switch is good enough, as its bigger engine (than say a 1.5 car) is giving higher engine braking.

If talking about Cameron Highlands, just D will do.

Not 1st gear la. You can't hardly drive in gear 1. 1st gear is getting the car moving from standstill, and quickly change to gear 2.

When engage into L (or S), you can still move up to 60-70 kmh. Which is gear 3 if in a manual car.

From the top of Genting going down to where there's the police guard house, this stretch is the steepest and most winding. I would use L (or S) on this stretch, and do less braking.

At the guardhouse where there got road bumps and you need to go very slow, this is where I would change back to D.