[swarnava.m]

Originally Posted by SDP

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.

I am sorry if this sounds like a stupid question, but wouldn't that vary with different factors? For example, right now I can think of two..

1. A/C being used, in which case the number of revolutions of the wheel would be lower than what it would be while the A/C is off.
2. Driving up an incline, where, again, the number of revolutions of the wheel would be lower than that on level ground, and exactly the opposite while driving downhill, in which case it would be more than that on level ground.

Correct me if I am wrong.

[SDP]

Originally Posted by swarnava.m

I am sorry if this sounds like a stupid question, but wouldn't that vary with different factors? For example, right now I can think of two..

1. A/C being used, in which case the number of revolutions of the wheel would be lower than what it would be while the A/C is off.
2. Driving up an incline, where, again, the number of revolutions of the wheel would be lower than that on level ground, and exactly the opposite while driving downhill, in which case it would be more than that on level ground.

Correct me if I am wrong.

The 2 situations you have mentioned are actually interesting. I did not mention those 2 and many others, so as not to complicate the discussion. With AC On and/or driving up-hill needs additional power from the engine, that can be achieved by increasing the rpm (assumption - the engine has not already peaked out) or downshifting to the right gear. When you need additional power, you need additional power and there might not be too much you can do about fuel-efficiency. If it means driving in the first gear to tackle a very steep ascent and revving heard, you have to do it, mileage be damned. Still by choosing the right gear and keeping the engine in the optimum RPM range, would get you the best possible fuel-efficiency in that "specific" situation.

[wildsdi5530]

@swarnava.m&@SDP; in the 2 situations mentioned, rpm does not increase. For a given gear, at a given speed, rpm is constant. The load on the engine increases and so does the fuel consumption.

[Aroy]

Originally Posted by swarnava.m

I am sorry if this sounds like a stupid question, but wouldn't that vary with different factors? For example, right now I can think of two..

1. A/C being used, in which case the number of revolutions of the wheel would be lower than what it would be while the A/C is off.
2. Driving up an incline, where, again, the number of revolutions of the wheel would be lower than that on level ground, and exactly the opposite while driving downhill, in which case it would be more than that on level ground.

Correct me if I am wrong.

If you do not ask questions, stupid or not, you will remain ignorant, which is even more stupid!

As stated in the previous post, at a given speed in a given gear the engine RPM will be same, irrespective of load. Hence there is no case for the wheel revolutions to be less under load, at a given speed. As the load varies, so does the amount of fuel required to maintain the engine RPM, hence even though you are in say the 4th gear at 60km/h, the fuel consumption will be more if you have the AC on, or are going up hill. Similarly if you are going down hill the fuel consumption will be lower.

What you are alluding to is that for a constant depression of the accelerator (constant fuel?), the speed will vary with the load.

[ash_blore]

Originally Posted by swarnava.m

I am sorry if this sounds like a stupid question, but wouldn't that vary with different factors? For example, right now I can think of two..

1. A/C being used, in which case the number of revolutions of the wheel would be lower than what it would be while the A/C is off.
2. Driving up an incline, where, again, the number of revolutions of the wheel would be lower than that on level ground, and exactly the opposite while driving downhill, in which case it would be more than that on level ground.

Correct me if I am wrong.

Definitely wrong. How can turning on the a/c have anything to do with the rpm? For a given speed and gear, the engine rpm will always be same irrespective of load. This is because the wheels are hard connected (via the various shafts) to the engine.

The reason for the reduced FE is that to get to this rpm, the engine has to work harder. Working harder means more fuel burnt.

Think about it, check it out with your own car rpm meter and the speed. The engine rpm will be same irrespective of everything else (a/c on/off uphill/downhill etc) for a given speed and gear combination.

[cranky]

Power output always has to be equal to or greater than the workload. Otherwise car will not move forward, or decelerate to a halt (eventually), or move backward.

Workload is a sum of all retarding forces on the car and the eventual impact is felt at the wheels. The only translation of workload and output is only translated into the 4 contact patches on the road, which take the entire load of the vehicle regardless of direction of travel, payload, power output or retarding force.

Every unit of power generated is transmitted to the wheel through the transmission (including differential, axle and bearings), and every unit of retarding force translates into a physical motion at the wheel. A car with no engine or braking power will slide in the direction of an incline, or stay motionless on absolutely level ground when all its 4 wheels are at equilibrum with gravity.

You may not realise this, but it takes effort to just stand still - humans, animals, or mechanical devices. A car with no suspension will tend to move in the direction of gravity. Even at standstill, the suspension is working, as are the tyres and the air inside them.

Now

The power output of the engine related to road speed is a critical factor in determining fuel efficiency. So if an engine is producing optimal volumetric efficiency (lowest consumption:output ratio), the road speed will determine the fuel efficiency at the wheel.

For example if an engine is the most efficient at 30BHP, when the engine is producing 30BHP in 1st gear you will be moving at (say) 10km/hr. When it is in 5th gear, it will move at 60km/h for the same power output. Obviously the latter case will cover maximum distance and deliver what we understand as fuel efficiency (erroneously so).

Of course, if the workload dictates more power is necessary given the amount of retarding force (either payload, or incline, or airconditioning) or desired forward momentum, then the engine has to operate outside its most efficient parameters. For example, a downshift may be required to a lower gear, or more accelerator input may be needed.

All parameters and combinations of forces only translate to a force at the wheel. The sum total of these forces determine the relationship between rolling resistance (in the tyre world this would be only tread resistance, it actually should include all other retarding forces) and forward momentum.

[swarnava.m]

Originally Posted by SDP

The 2 situations you have mentioned are actually interesting. I did not mention those 2 and many others, so as not to complicate the discussion. With AC On and/or driving up-hill needs additional power from the engine, that can be achieved by increasing the rpm (assumption - the engine has not already peaked out) or downshifting to the right gear. When you need additional power, you need additional power and there might not be too much you can do about fuel-efficiency. If it means driving in the first gear to tackle a very steep ascent and revving heard, you have to do it, mileage be damned. Still by choosing the right gear and keeping the engine in the optimum RPM range, would get you the best possible fuel-efficiency in that "specific" situation.

Originally Posted by wildsdi5530

@swarnava.m&@SDP; in the 2 situations mentioned, rpm does not increase. For a given gear, at a given speed, rpm is constant. The load on the engine increases and so does the fuel consumption.

Originally Posted by Aroy

If you do not ask questions, stupid or not, you will remain ignorant, which is even more stupid!

As stated in the previous post, at a given speed in a given gear the engine RPM will be same, irrespective of load. Hence there is no case for the wheel revolutions to be less under load, at a given speed. As the load varies, so does the amount of fuel required to maintain the engine RPM, hence even though you are in say the 4th gear at 60km/h, the fuel consumption will be more if you have the AC on, or are going up hill. Similarly if you are going down hill the fuel consumption will be lower.

What you are alluding to is that for a constant depression of the accelerator (constant fuel?), the speed will vary with the load.

Originally Posted by ash_blore

Definitely wrong. How can turning on the a/c have anything to do with the rpm? For a given speed and gear, the engine rpm will always be same irrespective of load. This is because the wheels are hard connected (via the various shafts) to the engine.

The reason for the reduced FE is that to get to this rpm, the engine has to work harder. Working harder means more fuel burnt.

Think about it, check it out with your own car rpm meter and the speed. The engine rpm will be same irrespective of everything else (a/c on/off uphill/downhill etc) for a given speed and gear combination.

Thanks to all of you. I understand now. It was my fault, I interpreted it wrongly. The engine delivers the same throughput at the same rpm, just that it has to work harder to get there.

Now this brings me to my other query. If 4th gear is considered as delivering a 1:1 ratio, (i.e. number of revolutions of the wheel=rpm), how can that stay constant for different cars? What I mean is, wouldn't that depend upon the size of the engine as well as the size of the wheels?
For example, consider a Honda Civic and a Maruti SX4. Both have R15 wheels. However, the civic has a much bigger engine and hence is a much powerful car. So, if I consider that at 4th gear 2000 rpm they both turn out a 1:1 ratio, wheels being the same size, both cars should run at the same speed. However, that is not the case.

Again, I might me missing a huge point somewhere, so please forgive me if I am being naive. I am just putting forth what strikes me.

[Aroy]

Originally Posted by swarnava.m

Now this brings me to my other query. If 4th gear is considered as delivering a 1:1 ratio, (i.e. number of revolutions of the wheel=rpm), how can that stay constant for different cars? What I mean is, wouldn't that depend upon the size of the engine as well as the size of the wheels?
For example, consider a Honda Civic and a Maruti SX4. Both have R15 wheels. However, the civic has a much bigger engine and hence is a much powerful car. So, if I consider that at 4th gear 2000 rpm they both turn out a 1:1 ratio, wheels being the same size, both cars should run at the same speed. However, that is not the case.

Again, I might me missing a huge point somewhere, so please forgive me if I am being naive. I am just putting forth what strikes me.

When you say 1:1 it is the ratio of the gear box input to gear box output. After that there is a reduction in the differential, any where from 1:3 to 1:5, (other ratios are there). So in effect the final ratio at the wheels is the product of the gear box and the differential ratios. If you have a transfer box you add another ratio that of the transfer box.

Thus the final ratio of engine to wheel RPM is different in every vehicle. This ratio is designed to give what the designer wants from the vehicle - maximum speed, or maximum acceleration or maximum FE. The ratio is again different for diesels as they have relatively lower top RPM, and Petrols, which are free wheeling and may go upto twice the RPM. For off roading you may want a high ratio so that you can crawl at low speed while the engine is in its peak power band - see discussions in the 4x4 sections on what ratios of differentials to chose.

Another factor is the actual circumference of the tyre. Just having 15" rims does not fix the circumference, as the extra height of rubber is determined by the product of rim width and the aspect ratio. Thus 4" wide rim with 100% aspect will have 2 x 4" = 8" height increase over the rim width, as will 8" wide rim with 50% aspect ratio, while an 8" width rim with 60% aspect ratio will increase the height by 2 x 8*.6 = 9.6". That is why to maintain the same circumference (and total width), wider rims have lower aspect ratio tyres (in the above example you will have to decrease the rim diameter by 1.6" to have 6-% aspect ratio on 8" width)

[swarnava.m]

Originally Posted by Aroy

Another factor is the actual circumference of the tyre. Just having 15" rims does not fix the circumference, as the extra height of rubber is determined by the product of rim width and the aspect ratio. Thus 4" wide rim with 100% aspect will have 2 x 4" = 8" height increase over the rim width, as will 8" wide rim with 50% aspect ratio, while an 8" width rim with 60% aspect ratio will increase the height by 2 x 8*.6 = 9.6". That is why to maintain the same circumference (and total width), wider rims have lower aspect ratio tyres (in the above example you will have to decrease the rim diameter by 1.6" to have 6-% aspect ratio on 8" width)

Absolutely!! I just completely overlooked that..!! Stupid me.

Originally Posted by Aroy

When you say 1:1 it is the ratio of the gear box input to gear box output. After that there is a reduction in the differential, any where from 1:3 to 1:5, (other ratios are there). So in effect the final ratio at the wheels is the product of the gear box and the differential ratios. If you have a transfer box you add another ratio that of the transfer box.

Thus the final ratio of engine to wheel RPM is different in every vehicle. This ratio is designed to give what the designer wants from the vehicle - maximum speed, or maximum acceleration or maximum FE. The ratio is again different for diesels as they have relatively lower top RPM, and Petrols, which are free wheeling and may go upto twice the RPM. For off roading you may want a high ratio so that you can crawl at low speed while the engine is in its peak power band - see discussions in the 4x4 sections on what ratios of differentials to chose.

Alright, couldn't really comprehend some of that, basically because of my lack of expertise in this field. But yeah, I get it. What you are saying is, the ratios that we are talking about here, are just local to the gearbox and for it to transfer to wheel revolutions, there are many other intermediaries. Well, makes sense!!

That really helped, thanks a lot.

[goandude]

Originally Posted by Astleviz

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.

Originally Posted by VeluM

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 found a graph to explain the fuel efficiency in each gear and various speeds. The lowest speed in top gear gave the best FE Golf GTI graph