Car Speed:  When driving at a speed of 40 to 50 kilometers per hour, you should use third or fourth gear,

Car Speed:  When driving at a speed of 40 to 50 kilometers per hour, you should use third or fourth gear, depending on your car model and the slope of the road.

Here are some things you should pay attention to:

Engine RPM: At a speed of 40-50 kmph, your engine should rotate between 2000 and 2500 RPM. If the RPM is too high or too low, you may want to change gears.

Engine noise: If your engine is under a lot of stress, you may want to change gears.

Car vibration: If your car is vibrating, you may want to change gears.

Here are some general guidelines:

30-40 kmph: 2nd or 3rd gear

40-50 kmph: 3rd or 4th gear

50-60 kmph: 4th or 5th gear

To increase mileage, you should try to drive in higher gears as fast as possible.

Here are some additional suggestions:

Accelerate and decelerate slowly: Avoid sudden acceleration or braking.

Use air conditioning sparingly: Air conditioning can increase fuel consumption.

Keep your tyres properly inflated: Underinflated tyres can increase fuel consumption.

Have your car maintained regularly: A well-maintained car is more fuel efficient.

It's important to remember that these are just general guidelines. The best way to know the best gear for your car is to experiment and see what works best for you.