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Old 9th November 2011, 11:57   #1
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Default Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

My ikon returns the best mileage figures when driven at 80 kmph in 5th gear which translates into approx. 2000 RPM. In 4th gear, 2000 RPM translates to approx. 60 kmph and in 3rd gear it is approx. 42 kmph.

Would I get similar FE figures if I drove the car at 80 kmph in 5th gear or 60 kmph in 4th gear or 42 kmph in 3rd gear or would it vary at different gear/speed combos assuming RPM is maintained at 2000 RPM?

Thanks in advance.
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Old 9th November 2011, 12:04   #2
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

It's all about how much Accelerator you push. But there is a catch.

One may like to attain as much Distance in as much low Acceleration i.e. as much low RPM in a way. So the equation is highest gear + lowest accelerator Or lowest RPM = Max FE. That's the reason you get highest FE on highways if the RPM/Accelerator push is low.

Hope this helps.
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Old 9th November 2011, 12:23   #3
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

So we press the accelerator more in 4th gear at 60 kmph (2000 rpm) as compared to 80 kmph in 5th gear (2000 rpm)? I'm not talking about accelerating to 60 or 80 kmph but about maintaining those speeds/rpm once I get there.
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Old 9th November 2011, 12:29   #4
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Quote:
Originally Posted by Vibhanshu View Post
It's all about how much Accelerator you push. But there is a catch.

One may like to attain as much Distance in as much low Acceleration i.e. as much low RPM in a way. So the equation is highest gear + lowest accelerator Or lowest RPM = Max FE. That's the reason you get highest FE on highways if the RPM/Accelerator push is low.

Hope this helps.

Highest gear and lowest RPM should return the best mileage. Also, higher the speed, lower the mileage due to air resistance. The air resistance at 120km/hr is much higher than that at 60km/hr.

EDIT: I am also confused now. Me too now looking for the right answer. The mileage difference with same accelerator level/RPM at 4th and 5th gears without caring speed.

Last edited by coolclouds : 9th November 2011 at 12:37.
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Old 9th November 2011, 12:35   #5
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Default

Quote:
Originally Posted by Astleviz
So we press the accelerator more in 4th gear at 60 kmph (2000 rpm) as compared to 80 kmph in 5th gear (2000 rpm)? I'm not talking about accelerating to 60 or 80 kmph but about maintaining those speeds/rpm once I get there.
At 2000rpm in 4th gear, the car is running at a slower speed than at the same rpm in 5th gear - 60 vs 80. Since fuel efficiency is the number of kms covered per litre, moving at a greater speed at the same rpm means greater FE. Maintaining 60 in 4th would mean the same engine efficiency, but the vehicle's Milage will be lower. Hope that helps.
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Old 9th November 2011, 12:42   #6
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Quote:
Originally Posted by VeluM View Post
At 2000rpm in 4th gear, the car is running at a slower speed than at the same rpm in 5th gear - 60 vs 80. Since fuel efficiency is the number of kms covered per litre, moving at a greater speed at the same rpm means greater FE. Maintaining 60 in 4th would mean the same engine efficiency, but the vehicle's Milage will be lower. Hope that helps.
Ok, the engine efficiency is the same but one travels more at 80 kmph for the same amount of fuel. That makes sense, Thanks.
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Old 9th November 2011, 12:44   #7
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Am driving an ANHC which has an instant mileage calculator bar. As per the info provided, the best mileage is provided in 5th at 1500-2000 rpms. coming very close is 4th at similar rpms.

But no matter what rpm, the mileage is always low (almost 50%) on 1st, 2nd and third ie its pointless to keep rpms low and steady foot in 1,2,3 gears expecting similar mileages as in 4th/5th.
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Old 9th November 2011, 12:47   #8
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Quote:
Originally Posted by VeluM View Post
At 2000rpm in 4th gear, the car is running at a slower speed than at the same rpm in 5th gear - 60 vs 80. Since fuel efficiency is the number of kms covered per litre, moving at a greater speed at the same rpm means greater FE. Maintaining 60 in 4th would mean the same engine efficiency, but the vehicle's Milage will be lower. Hope that helps.
I have a small question here.

Case 1: Car in 5th gear, RPM 2000, speed - 80 km/hr
Case 2: Car in 4th gear, RPM 1800, speed - 60/km/hr

Which one will return better mileage? (pardon me if its a nonsense question)

Edit: Thanks, above post gets me the answer.

Last edited by coolclouds : 9th November 2011 at 12:51.
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Old 9th November 2011, 13:00   #9
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Quote:
Originally Posted by Vibhanshu View Post
So the equation is highest gear + lowest accelerator Or lowest RPM = Max FE.
Quote:
Originally Posted by coolclouds View Post
Highest gear and lowest RPM should return the best mileage.
Not necessarily. There is limit for this lowest RPM, below which you will be lugging your engine, and consequently, the FE decreases.

Quote:
Originally Posted by coolclouds View Post
Case 1: Car in 5th gear, RPM 2000, speed - 80 km/hr
Case 2: Car in 4th gear, RPM 1800, speed - 60/km/hr

Which one will return better mileage?
Well, it varies from car to car. As, different engines have different power torque curves. Also, the drag coefficient need to be considered (Not that evident at these speeds).


IMO, for attaining the best FE, one has to locate the sweet spot for his car, when the engine is most relaxed. Couple that with the tallest gear and you get the best combo.

Also, factors like weight, sustaining the momentum efficiently, tyre air pressure, outside temperature, AC usage affect FE.
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Old 9th November 2011, 13:06   #10
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Great topic. While I wait for experts to dissect this and enlighten us further, my understanding is it's always a function of the total RPMs run in a journey, from start to end. Meaning, suppose the length of the journey is 1 km.

If you do this driving normally, i.e., right gear for right speed whatever that is, the mileage is x.

If you "over-revv" the engine, higher speeds in lower gears or even while lugging, you increase total RPMs done by the engine, so your mileage is x-y.

Few other "usages" also can directly be clubbed to the RPM: a/c, blower, stopping and starting, hard braking & starting etc., hence in my understanding, the efficiency becomes a function of RPM.

Now what's the beauty of cars with overdrive?
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Old 9th November 2011, 13:22   #11
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

I remember having attempted to answer this earlier

Fuel consumption is a combination of three ratios:

1. Throttle position compared to current engine speed.

2. Engine speed compared to road speed.

3. Road speed compared to aero drag+rolling resistance.

Max fuel consumption usually takes place when any or all of these ratios are very high. As the ratios lessen the consumption reduces.

So if you're moving from standstill with engine at idling, the amount of throttle you have will determine the instantaneous consumption.

Similarly at very high road speed the drag and rolling friction will be very high.

At highway cruising speeds the throttle is usually held at a single position and the car is at equilibrum, leading to the highest fuel economy. Hypermiling is a combination of this knowledge and some terribly unsafe techniques to squeeze every last drop of fuel economy.

For regular driving, I wouldn't worry, just use a little less throttle than you would, but keep the engine in the right rev band. In city driving you have very little control over the fuel consumption anyway, it's really the highways where you can get away with trying to keep economy down.

Last edited by cranky : 9th November 2011 at 13:32. Reason: typo :)
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Old 9th November 2011, 13:42   #12
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Quote:
Originally Posted by cranky View Post
Max fuel consumption usually takes place when any or all of these ratios are very high. As the ratios lessen the consumption reduces.

So if you're moving from standstill with engine at idling, the amount of throttle you have will determine the instantaneous consumption.

Similarly at very high road speed the drag and rolling friction will be very high.
Nice explanation!

Just thinking out loud, won't the other factors you've mentioned directly or indirectly translate to more/less load, hence more/less RPMs?
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Old 9th November 2011, 14:11   #13
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

very good question!

Quote:
Originally Posted by VeluM View Post
At 2000rpm in 4th gear, the car is running at a slower speed than at the same rpm in 5th gear - 60 vs 80. Since fuel efficiency is the number of kms covered per litre, moving at a greater speed at the same rpm means greater FE. Maintaining 60 in 4th would mean the same engine efficiency, but the vehicle's Milage will be lower. Hope that helps.
This is the most accurate answer in my opinion. The engine's efficiency is highest for a specific range of rpms. The gearing system translates engine's revolutions to the wheel's revolutions. The gearing is more efficient in higher gears where it is generating maximum no of wheeel's revolutions for the same no. of engine's revolutions. Combining these two, engines rpm where it is most efficient and the highest gear would result in the best fuel efficiency.


Quote:
Originally Posted by revintup View Post
Am driving an ANHC which has an instant mileage calculator bar. As per the info provided, the best mileage is provided in 5th at 1500-2000 rpms. coming very close is 4th at similar rpms.

But no matter what rpm, the mileage is always low (almost 50%) on 1st, 2nd and third ie its pointless to keep rpms low and steady foot in 1,2,3 gears expecting similar mileages as in 4th/5th.
As per my understanding, 4th gear delivers 1:1 rpm. i.e. for the engine's one revolution, you will get one revolution of the wheel in the 4th gear. In 5th gear, you are in over-drive and for each revolution of the engine, you get more than 1 revolution of the wheel. I hope that should explain why 4th and onwards gears would give better efficiency and not 1-2-3.

Last edited by SDP : 9th November 2011 at 14:18.
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Old 9th November 2011, 14:14   #14
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Any engine will probably have a comfor range in terms of rpm where it returns the best FE. However, what really determines the actual FE is how much fuel is used up getting to that rpm. If the engine is presented with a higher load (either more people sitting in the car or by virtue of driving on an incline) then more energy (fuel) is required to get to that comfort rpm range. This is what really determines the actual FE.
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Old 9th November 2011, 14:17   #15
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

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Originally Posted by dhanushs View Post
Not necessarily. There is limit for this lowest RPM, below which you will be lugging your engine, and consequently, the FE decreases.
Acceleration just enough to have the momentum going. This will also depend on the ECU mapping/Gear Ratios.
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