gear ratio and fuel economy

[wilem]

Originally Posted by Iwannacamp

Ok...I from Arkansas. I talk slow, understand nothing and skip stuff when I read....so if I got anything correct then there are a few factors for fuel consumption basics. Friction:tires and weight maybe. Wind resistance: faster is more. Torque:horse power to weight/gear ratio. Somehow this math has to work for your hp/weight/fuel use.

Is this even close?

sounds good to me.

[Al Fresco]

JFM-jr:

As I have repeatedly stated, I am taking issue with an earlier claim that a lower gear ratio requires a higher engine rpm to maintain a given speed. I agree with you that a gear ratio of “4.56 will need alot more [engine] revolutions to maintain 60mph than a 3.73 will..” Therefore, is the converse not also true, i.e., that a gear ratio of 3.73 will need a lot fewer engine revolutions to maintain 60 mph than a 4.56 will? If so, that contradicts the earlier claim I am addressing.

Of course the wheels/axles rotate at the same rate to maintain 60 mph regardless of the gear ratio. What I have been arguing is that, with a lowered gear ratio, the engine rpm must be lowered (not increased) to maintain that speed, all other factors being equal. Are you in agreement? If so, then you are, like me, in disagreement with the earlier claim.

Not to belabor the point, but what is faulty about my understanding that, if the gear ratio is increased to 4.56, then for a given engine>transmission>pinion rpm the ring-gear>>wheels will make fewer revolutions than with a 3.73 gear ratio? I am not ignorant of the fact that the engine>transmission>pinion rpm varies as a function of how far the gas pedal is depressed. I am just wondering what you think is wrong with my understanding of the relationship I as presented it.

[garbonz]

Your understanding is wrong because it is wrong. Simply put, a higher ratio rear axle ratio requires a higher engine speed to maintain the SAME SPEED down the road. That is a fact whether you understand it or not.

If your truck runs 3000 rpm with a 3.42 axle ratio at 60 mph, it will run at 3271rpm with the 3.73 gears.

Higher numerical gear ratios will require higher engine rpm at the same speed.

The higher ratios may result in reduced stress on the power train, but it depends on other variable, such as torque curves and transmissions shift points.

THESE ARE FACTS NOT OPINIONS.

[youroo]

Originally Posted by wilem

more than twice as much, that front diff is a whole lot more involved than the rear. on say a K-5 or more current IFS the front labor is quite a bit more than the rear.

Post # 21 covered that! Youroo!!

[prairiecamper]

Originally Posted by wilem

Nothing wrong with that, I try to tow in a place that suits the current gear selection. There is a window for each gear, I don't like to sit on the fence of at say 55. When 5 mph more or less is easier on the system. I say system because I am a transmission guy I watch my trans temp closely. You need for the converter to be locked for the trans to cool down. ( when a trans goes into hot mode it applies the TCC. To help it cool down)Whatever makes it stable is what I do. The new diesel I have has much much wider windows of operation. And that is simply because of more torque. The Allison transmission is a beast compared to what I was using. The rear axle and brakes are much more stout as well. Happy truck = happy trip.

I watch my tach, and I believe I can tell when my torque converter is locked, since you are a transmission guy is this not possible?

[BigBaron]

Wilem - is there a way to force the tranny to lock the torque converter? My expertise is manual transmissions, so I'm curious if a switch or ??? could be added. Also, where is a good place to learn about transmissions/drivetrains?

[JFM-jr]

Converter operation and the mechanics inside it is a really neat thing to learn about.

[SaskCampers]

Not sure on other makes and models, but the Super Duty Fords from 2011 on have a new 6 speed (same tranny gas or diesel) and the lockup is very pronunced and you can see it on the tach. Because there is a gear indicator you know if it was a gear shift or lockup and they lockup quite early especially in T/H or full manual mode. T/C lockup is a function of the factory programming of the tranny.

[Al Fresco]

Garbonz (assuming your post 43 was addressing me):

I agree with you that “higher numerical gear ratios will require higher engine rpm at the same speed.” Can you cite anything I have written that argues against that relationship?

If “higher numerical gear ratios will require higher engine rpm at the same speed,” then it follows that lower numerical gear ratios will require lower engine rpm at the same speed.

For the umpteenth time, I have been responding to an earlier post (number 7) that claimed “the lower your gear ratio, the greater the RPM's from your engine to maintain an equal driving speed. “ Am I correct in assuming we both agree that claim is not accurate, all other factors being equal?

[dustman_stx]

Originally Posted by Al Fresco

Garbonz (assuming your post 43 was addressing me):

I agree with you that “higher numerical gear ratios will require higher engine rpm at the same speed.” Can you cite anything I have written that argues against that relationship?

If “higher numerical gear ratios will require higher engine rpm at the same speed,” then it follows that lower numerical gear ratios will require lower engine rpm at the same speed.

For the umpteenth time, I have been responding to an earlier post (number 7) that claimed “the lower your gear ratio, the greater the RPM's from your engine to maintain an equal driving speed. “ Am I correct in assuming we both agree that claim is not accurate, all other factors being equal?

Your claim is inaccurate. A LOWER ratio is a higher numeric value. So, 3.73 gears are LOWER than 3.55 gears.

[wilem]

Originally Posted by BigBaron

Wilem - is there a way to force the tranny to lock the torque converter? My expertise is manual transmissions, so I'm curious if a switch or ??? could be added. Also, where is a good place to learn about transmissions/drivetrains?

before OBD II i would say it would be easy, now not so much.

the computers are so dynamic these days they are very Hard to fool. you would almost guarantee yourself a CEL/SES light.

you may be able to through a programmer. you loose all torque multiplication as far as the converter is concerned but that is why it cools when locked.Because a damper plate splined to the input shaft locks to the converter shell itself thus bypassing the guts.(impeller, stator and turbine) it would be similar to being in 6th gear in a manual and not down shifting. It almost has to unlock to climb any kind of hill. but diesels have so much torque you can get away without a dis-engagement. On a gas engine, it would really kill the power. and they are likely limited to 1:1 in the HOT MODE scenario.

[wilem]

Originally Posted by youroo

Post # 21 covered that! Youroo!!

this is getting no where, the guy asked what would be the difference in selecting 3.92 over 3.55 or something close to that over 3.21. IT WILL NOT BE TWICE AS MUCH. the labor for the job will be the same. he will MAYBE have to pay a few dollars more (most likely not but maybe) for the different higher ratio maybe even less if they are more popular and more available. you say it will be twice as much, and to you i ask twice as much as what?? does he have two trucks one 4x4 and one 4x2???

[youroo]

Originally Posted by wilem

this is getting no where, the guy asked what would be the difference in selecting 3.92 over 3.55 or something close to that over 3.21. IT WILL NOT BE TWICE AS MUCH. the labor for the job will be the same. he will MAYBE have to pay a few dollars more (most likely not but maybe) for the different higher ratio maybe even less if they are more popular and more available. you say it will be twice as much, and to you i ask twice as much as what?? does he have two trucks one 4x4 and one 4x2???

First off I DIDNT say twice as much! I made a statement that if HIS truck is a 4X4 that it would be a Big diff. in price!
READ POST # 21! Youroo!!

[wilem]

great advice everyone. so maybe I'll go for the 3.92 ratio as that is as low as I can go. There is only like $10.00 difference in price between the 3.55 and the 3.92.

Originally Posted by youroo

If your truck is 4x4 there will be a BIG diff.in Price! Youroo!!

and for the last time, NO there will not be a big difference in price. from 3.55 to 3.92 because he already has his price in hand

unless you intent is that he trades trucks and then gets the same job done on a 4x4 and he now has a 4x2

DONE HERE

[Al Fresco]

Am I wrong in concluding that the term “gear ratio” is being used with two different meanings in these discussions?

On the one hand, there is the numerical value that can be calculated by dividing the number of teeth on the driven gear (the ring gear) by the number of teeth on the driving or input gear (the pinion). That is the specific meaning I am using. In this case, a lower gear ratio value results from the fact that there are fewer teeth on the ring gear, all other factors being equal.

On the other hand, “gear ratio" also appears to be used by some participants to refer to the condition that exists as a function of a particular arrangement of gears. In this case, a lower “gear ratio” is used to describe an arrangement that results in what is generally referred to as “lower gearing,” i.e., more power to the wheels with the wheels turning at a slower rate for a given engine rpm. In this case, a lower “gear ratio” actually results from the fact that there are more teeth on the ring gear and the calculated gear ratio is higher, all other factors being equal.

In one case, the meaning is prescriptive and refers to the cause – the calculated gear ratio. While in the other case, it is descriptive and refers to the general effect – the resulting so-called lower (or higher) gearing.

Assuming my conclusion is correct, it would be helpful in these discussions if those who comment would let us know which of these two different meanings they are using when they talk about gear ratios. This would be especially helpful for those, such as myself, who are relative novices when it come to this subject. Otherwise we can easily end up talking past one another.

[garbonz]

Originally Posted by Al Fresco

Garbonz (assuming your post 43 was addressing me): I agree with you that “higher numerical gear ratios will require higher engine rpm at the same speed.” Can you cite anything I have written that argues against that relationship? If “higher numerical gear ratios will require higher engine rpm at the same speed,” then it follows that lower numerical gear ratios will require lower engine rpm at the same speed. For the umpteenth time, I have been responding to an earlier post (number 7) that claimed “the lower your gear ratio, the greater the RPM's from your engine to maintain an equal driving speed. “ Am I correct in assuming we both agree that claim is not accurate, all other factors being equal?

Yepper's you is right.

[wilem]

Originally Posted by Al Fresco

Am I wrong in concluding that the term “gear ratio” i?

On the one hand, there is the numerical value that can be calculated by dividing the number of teeth on the driven gear (the ring gear) by the number of teeth on the driving or input gear (the pinion). That is the specific meaning I am using. In this case, a lower gear ratio value results from the fact that there are fewer teeth on the ring gear, all other factors being equal.

On the other hand, “gear ratio" also appears to be used by some participants to refer to the condition that exists as a function of a particular arrangement of gears. In this case, a lower “gear ratio” is used to describe an arrangement that results in what is generally referred to as “lower gearing,” i.e., more power to the wheels with the wheels turning at a slower rate for a given engine rpm. In this case, a lower “gear ratio” actually results from the fact that there are more teeth on the ring gear and the calculated gear ratio is higher, all other factors being equal.

In one case, the meaning is prescriptive and refers to the cause – the calculated gear ratio. While in the other case, it is descriptive and refers to the general effect – the resulting so-called lower (or higher) gearing.

Assuming my conclusion is correct, it would be helpful in these discussions if those who comment would let us know which of these two different meanings they are using when they talk about gear ratios. This would be especially helpful for those, such as myself, who are relative novices when it come to this subject. Otherwise we can easily end up talking past one another.

yes, it is being used with two different meanings in these discussions. It is not uncommon

you seem to have it clearly understood, it is just not understood equally by others. But the reason could be attributed to Low gear in the transmission is first gear or granny gear which is 1. High ratio, or with a transfer case being Low range 4, (way higher ratio) so people with or without a thorough understanding refer to a higher a ratio gear as low.... clear as mud?? hope it helps.


i hate to add this BUT (in reference to trannys only)
it can be under driven or over driven, either way the ratio is not directly relative to the reference of high gear and low gear..

[garbonz]

Looking at all the posts in context of the OP's questions it is apparent that the confusion is around terminology with low gear ratios creating higher engine rpm's and vice versa.

Simply put, I think the questions are answered, and that is that. I'm tired

[Al Fresco]

Thanks garbonz and wilem for confirming that two different and conflicting meanings for “gear ratio” are being used in these discussions. I was beginning to think I had slipped a gear myself!

[kandl]

Originally Posted by kwbybee

My question is if I switch the gear ratio out to 3.92 or 3.55 will my miles per gallon improve significantly. I'm hopping for 10 mpg into wind and 14 with no/little wind.

I would opt for the 3.92 gears - that ratio works half decently with the 5-speed RFE trans. We've averaged about 21 L/100km (10+ US MPG) overall while lugging the 7500 lb Rockwood some 18K km. And that includes lots of Rocky Mountain climbing as well. Unloaded the truck gets 14 L/100km (~17 US MPG) combined city/hwy. Hwy, unloaded and not making like a banshee, I've seen 11 L/100km (21 US MPG).

Safe travels!