# Driving at 45-55 kmph, in correct gear & fuel savings

PCRA aka Petroleum Conservation Research Association has been promoting driving at the speeds of about 45-55 kmph and says that it will result into savings of about Rs. 25,000 per year. I searched CE for discussions on the relationship between driving speeds and fuel economy; but couldn't find any discussion that addresses the issue.

There are other points raised by PCRA and it includes driving in the correct gear. I'm curious to know-
• How does the speed of 45-55 kmph result into saving of fuel? (Would appreciate sample calculations)
• How does driving in the right gear save fuel? - I'm assuming that using lower gear delivers more power to the vehicle but the fuel consumption goes high. But would appreciate more details on this.
Looking forward to responses.

## Replies

• Ramani Aswath
An IC engine performance is optimum at certain torque RPM conditions. This data has been known for long. I have seen this in early sixties (speed about 30 mph). At high gear and this speed the engine RPM and torque are the best thermodynamically. I am surprised that with all the advancement in IC engines, this does not seem to have changed. Well, thermodynamics also has not changed I guess.
• Saandeep Sreerambatla
I am driving bike since 15 years now and car since 2 years.
Driving in correct gear , I know most of the people know it. But the speed at which you are going and the gear you are using.

This also includes many things, if you are just starting the vehicle then put it in gear1, then slowly go to the top gear.

When you are slowing down your vehicle you need to go to a lower gear and drive.

Let me give you an example, if you are driving your bike in gear 3, and you had to slow down because of some bad road condition (most common in India).

Then you have two options, one is to shift to second gear or else, you can hold the clutch and accelerate more so that the vehicle doesnt get turned off and drive on.

So in the above scenario shifting to gear 2 is optimal and the other case is not.

This happens and we do it many times.
• Kaustubh Katdare
I remember reading that in 4-wheelers, if you maintain the RPM at about 1440; then you can have the max efficiency. How do we know that the RPM of 1440 corresponds to speeds of about 45-55 kmph? I think in car it's about 60-62 kmph (will have to check).
• Sarathkumar Chandrasekaran
Kaustubh Katdare
I remember reading that in 4-wheelers, if you maintain the RPM at about 1440; then you can have the max efficiency. How do we know that the RPM of 1440 corresponds to speeds of about 45-55 kmph? I think in car it's about 60-62 kmph (will have to check).
You can easily find the best efficient speed in the speedometer and do check the green range which represents the speed to obtain optimum efficiency.

It's little bit tricky to explain but I would try my best to answer it.RPM does not mean the revolution per minute of the wheel and it denotes the rpm of the engine.Higher the rpm denotes harder working of the engine.You would have also noted that in all specifications of cars and bikes while explaining about the power and torque, they would have quoted that "x" hp in "y"rpm or "u"Nm torque in "w' rpm.It all denotes the engines rpm.

The engines power is transmitted through clutch then followed by gearbox, propellor shaft,differential and to wheels.
following link helps to calculate the speed from rpm :Speed versus RPM calculator
• Sarathkumar Chandrasekaran
Also do remember that gradual up shifting and down shifting helps in improving the efficiency and life time of the gearbox.Over throttling in a short period can increase fatigue of the engine.
• Anoop Kumar
Observations:
A.V.Ramani
An IC engine performance is optimum at certain torque RPM conditions.
+1
RPM meter (Revolution per *minute) which denotes Engine Piston RPM is the main key factor whether you are getting optimal fuel efficiency (not the mileage) or not.
• Sarathkumar Chandrasekaran
ianoop
Observations:

+1
RPM meter (Revolution per meter)
IS IT revolution per minute or revolution per meter?
• Anoop Kumar
SarathKumar Chandrasekaran
IS IT revolution per minute or revolution per meter?
TypoðŸ˜–

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