Are these not available? Anybody know why not?





Michael

I know bassinrver (sp) put some in his truck, but I don't think they can be ordered that way.

You can have a 4.10 installed, not sure why you would want to do thishttp://www.dieselplace.com/forum/smileys/Ermm.gif, but I know for a fact your fuel mileage will go to hell. Buddy did that to his and it dropped from 20 most of the time down to about 12 mpg. So something to think about. Also you have to understand that if you have a 4WD and you change one you have to change the other because if you do not then you will have issues should you ever put it in 4WD.

Basically you only get one gear ratio because the diesel doesn't have the RPM range that a gasser has. So unless you've got more than five gears to work with, cruising speed would be compromised especially in the MPG area because the diesel is then running too close to it's max RPM.


You got gobs of torque with the Dmax, just use it.....http://www.dieselplace.com/forum/smileys/Approve.gif

I went as far as calling GM. I tried to get the factoy to put them in when I oreded mine and they said nope! So I had 4.56's put in when I did my lift.

All of your responses make sense. Thanks for all the input. So you guys with 37 inch wheels just cruise in 5th at 1400 RPM?


I will be using H2 takeoffs at 34.5", that is why I asked. 15% more circumference=15% less RPM, maybe out of the power band somewhat.


What is the logic capability of the allison? Does it "learn"?

My $.02 anything larger than 35 you should regear. I have a buddy that has 35's and 3.73's he has no problems with his.

Just put 12" wheels on it. That will lower the ratio.

I don't know how to do the math on this but what is the final drive ratio with 245 and 3.73 vs. the 315 and 4.10. Would be good to know this. Thanks.


LA DMAX

your money is better spent on a programmer like the juice to make up for the bit of low end delay you might experience. i am running the 315s and like where the rpms are at more now than with the 245s. highway crusing is at 1900 rpms most of the time.

4:10 gearing was listed as an option when I bought mine, can't understand why you can't order it with that. But then, can't understand why any one would want it unless they had some big rubber on the truck either.

If you run tall tires such as 34" or taller, and you don`t change to lower gears your mileage will suck.


37`s with 3:73.1 gears will get much worse mileage then if you were to swap out to 4:56.1`s


My last truck a 1/2 ton with a 350 V-8 gas engine got under 9 mph with 37`s and 3:73.1 gears. I swaped them out for 4:88.1 gears and my mph went up too 14+ under the same driving habits. I know Diesels are different, but not that much.

Any engine has various parameters (HP, torque, efficiency) that you can graph. Graphing these parameters vs. RPM produces curves. Overlaying the HP, torque, and efficiency curves, you should be able to determine the engine's "sweet spot" (the point where you get the best combination of power and efficiency). This is the RPM you typically want to run in your highest gear at highway speeds. I believe that I heard that the factory determined that point to be around 1800 RPM but, hopefully, someone here will chime in with an actual verifiable number...


In the case of larger diameter tires, you are changing the effective leverage your engine has to propel the vehicle. If the change is big enough, you could lower the RPM's enough that you require more throttle (fuel) to to produce the necessary power to accelerate the vehicle and maintain a given speed. Since you're using more fuel, your mileage goes down.


A general rule of thumb in the 4wheeling community is that more than a 10% change in tire diameter dictates a gear change to stay close to your previous performance.


Just my $0.02


Mike

Very well said sloth!


That is what I was trying to say, but you said it much better. And yes the Duramax/Allison run best at about 1800 rpm.


Another thing to look at is, if you go to far on the tires you may end up running in 4th gear more then 5th. At that point it would be wise to go to lower gears. (higher number)

Well, I found out how to compare gear ratios and tire size, I think. I found it in this months Peterson's 4wheel&offooad.


336 X [(3.73 x 65mph x .71/30" tires)] = 1928 rpm


336 X [(4.10 x 65mph x .71/35" tires)] = 1816 rpm


Seems like at 65mph with 4.10 and 35" tires we would be at that desirable 1800 rpm that the dmax likes. I don't know if other factors are involved that may change this info, or if I even did this right, but it looks good on paper.


LA DMAX

LA DMAX


Could you scan and paste that article here? You may have gotten onto something, but I can't make any sense of your equations, not to say they aren't correct, I just need the education.





What does "336" and ".71" represent?

masterp2,


The 336 is a constant that is used for that equation. The .71 is the overdrive ratio of the Allison trans (5th gear), 3.73 = rear gear ratio, 65 mph, 30 = tire diameter.


This might make it clearer:


[(3.73 x 65 x .71)/30] x 336= 1928 rpm http://www.dieselplace.com/forum/smileys/Geek.gif


JP


JP

Without any more explanation, what I draw from this is that at 65 MPH, the engine will be turning 1928 or 1816 RPM, depending on tire circumference (which is proportional to diameter) and gears.


Am I missing something? How does this address the RPM you should look for max efficiency, when tires and gears are changed? Maybe they were not intended to. As far as I can see, these equations are conversion formulas, but don't suggest how you should "align" tires with gears, i.e. to keep the power curve unchanged, the question is, what gears to use with 35 inch tires to maintain the same performance on the power curve.


In my opinion, the answer to that is easy. The ratio of the gears should be equal to the tire diameter ratio.


35/30=x/3.73, here x=4.35, so 4.35 gears on 35 inch tires will supply the same torque (curve) as the stock settup.


Without the new gears the truck will run "taller". Lower torque WILL be experienced throughout the entire band and in ALL gears. Simple physics. Since power and fuel consumption typically have an inverse relationship, and for the highway drivers mileage will increase if the new tires do not have significantly more mass (never the case) or higher road resistance (possible). But there can be no mistake, you will lose pull if a gear change does not accompany bigger tires. So in my example above, even the 4.10 gears are not enough for a 35 inch tire if the goal is to preserve the power curve. In my mind, the 4.10 gears would be preserving performance AND gas mileage. The idea that MPG would go down with higher gears makes no sense unless you can argue that putting on bigger tires on stock gears makes MPG go up.http://www.dieselplace.com/forum/smileys/Wacko.gif


The next question might be, "how much am I going to hurt performance by just changing tires?" Does anybody have thoughts on that or anything else here? This is just my understanding of things not gospel. Jump in if I'm wrong.


Michael

Here is how you figure out what gears you need if you change tire size.


Equivalent ratio= (new tire diameter/old tire diameter) x original gear ratio


Example....(35 divided by 30) times 3.73 = 4.35


4.35 would be the gears you would need to have a truck with 35's have the same gearing as a stock truck.

Isn't that what I said?


Sorry, I'm a bit bitchy after getting off the phone with an idiot who just did the same repair on my bike for the 3rd time, and guess what, it still didn't solve the problem.

HDMAX-


Thanks for the kudos eventhough, I was just eloborating on your original post!http://dieselplace.com/forum/smileys/Big%20Smile.gif


I have to also agree with you that D/A owners must be careful with the D/A and overall gear ratios not just because of the performance/mileage issues typical of any engine but because if your RPM's end up at an "odd" spot, the Allison will constantly hunt between 4th & 5th. Yet another reason to apply the 5th gear lockout mod and keep your overall gearing within 10% of stock.


-Mike

masterp2,


I'm embarassed to say that I don't have a scanner, not even at work, but this is what the formula is:


336 X [(axle ratio x vehicle speed in mph x overdrive ratio)/tire diameter in inches] = engine speed in rpm


Just like heartbeat said 336 is a constant and .71 is the allison 5th gear ratio.


Idealy if you want to keep the same engine rpm at a given speed you would follow the ratio, your example showed a 4.35 gear. I would like to drop a few rpm when at 65 or 70 mph, seems like it would get better milage, at least when unloaded. Personally I would like to be at 1850rpm's at 65mph.


The whole milage thing with bigger tires is this, in general: bigger tires may give you better mpg's if you do alot of freeway driving, because your rpm's will be lower. The down side is that if you do alot of stop and go driving it will take alot more fuel and power to turn the tires because your torque has been reduced. Sorry if you know this stuff already, I'm just being long winded.


The problem with these differentials is that there are not a whole lot of ratios available yet. The 4.10 are readily available because of the option on the 8.1's. I don't think you will find 4.35 any time soon. I know 4.56 is out there, but if you are not running 37's, it might be too much gear.


If I was going to bump up to a 35" tire, I would consider changing the ratio, as long as the allison was ok with it. You should be able to get a 4.10 from a dealer or even a take off from someone who has an 8.1 that is going up to 4.56. I just might be asking shops if they have any customers doing this and if they want to sell their ring gear.


LA DMAX

masterp2 and everyone else,


First, I must apologize for this essay length post but, I am honestly trying to help...


masterp2, to answer your last post, 1bad01 did basically reiterate your conclusions. But, 1bad01 (and you, for the first half of your post) are actually correct.


Here are some helpful formulae showing how HP, torque, and RPM are directly related:


T = (HP*5252)/RPM or HP = (T*RPM)/5252


5252 is a constant. I won't waste space here with the details. A good explanation of HP can be found here: http://auto.howstuffworks.com/horsepower1.htm and torque is here: http://science.howstuffworks.com/fpte3.htm


The point is that HP & torque are directly related to each other and are both dependent on RPM. So, if you want to stay as close as possible to the stock power & performance, you want to stay close to the stock RPM at a given speed in 5th gear or, as 1bad01 said:


new gear ratio = (new tire dia./old tire dia.) x stock gear ratio


Regarding efficiency (mileage), I feel I should point out a few factors:


First, the rolling resistance of a larger tire is usually greater due to the obviously larger area of the contact patch , which is often inadvertently increased, even more, by the owner lowering tire pressures in an effort to regain stock ride quality.


Second, while it may seem negligible, air resistance goes with the square of speed. So, increasing the height of a vehicle even an inch or two greatly increases its overall profile from the ground (a.k.a. increased drag). It does not seem to be too big of a deal until you think about the increased profile area times the square of the speed. At 65 MPH, the increased resistance due to the extra area really adds up!


Third, while a slightly larger tire often does not weigh much more than a stock tire, it is not the overall weight that matters when calculating inertia (for our purposes here, inertia is resistance to torque). What matters is how much weight at what diameter? If you have 2 tires that weigh exactly the same but one has a diameter of 30" and the other has a diameter of 35", the 35" tire has approximately 55% more inertia (assuming an even distribution of material) than the 30" tire even though the difference in diameter is only about 15% (**note: these are approximations -- again, I will not waste space here; google for the inertia of a wheel, find the correct formulas, and do the math. You will be surprised).


<SPAN style="FONT-SIZE: 8pt; FONT-FAMI

Well written an insightful.

I tow 5% of the time, and the rest of the time, I like between 70-80 cruising.


I am getting some reports that 4.10 is now available on the D/A. If so it may still not be a good idea for me, so I probably won't worry about it.


Sloth and LA Dmax, I think your explanations are well put. I've heard it said that fuel economy in the diesel is all about the RPM sweet spot, after which you lose %. For someone who like to go 80, some increase over the 245's would likely be quieter and more economical.


This changes the picture a little, when considering tranny stress. The bigger tires do place more work on the transmission, 5th gear coming to mind specifically. You know how for non-commercial towing gassers, the sticker that reads, "don't tow heavy loads using overdrive". With bigger tires, this may be the biggest consideration.


What is this OD lockout I have heard about?


Thoughts?

masterp2:


My bad, I must have had my eyes closed the 1st time I read through the topic.