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

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You can have a 4.10 installed, not sure why you would want to do this
, 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.
 

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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.....
 

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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.
 

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Discussion Starter #6
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"?
 

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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.
 

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

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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
 

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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.
 

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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.
 

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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.
 

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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
 

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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)
 

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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
 

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Discussion Starter #16
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?
 

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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



JP


JP
 

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Discussion Starter #18
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.



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
 

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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.
 

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Discussion Starter #20
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.
 
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