I was just wondering...

As I never tow anything and am not planning on, and would actually prefer to not have the ability to in order to avoid anybody asking me to help them move or such, I was thinking about changing out my rear drive gear.


Now, the questions I have are (A) how do you know what drive ratio you have, (B) what kind of gears are available for my truck and (C) how low of a drive ratio could you go on these trucks before you start hurting 1/4 mile times.


The third one is the one that gets me thinking. Muscle-cars would usually pull down the best 1/4 times with big ratios, some got above 5:1 even. But since these trucks got a bigger torque-HP ratio, and redline a lot lower, what is the best drive gear? Anybody know if it's possible to calculate the most efficient final drive gear based off of redline/torque/weight?

Just a thought.

PS. Did they make LSD's for these trucks?

 

A)there should be a tag on the diff cover...if you have the 3sp auto, you will have the ultra-rare (read--expensive to work on) 3.07:1 gears. They are about as 'anti-towing' as you can go, to be honest. Thing is, the Cummins doesn't care---it'll just yank the load anway!

b)the sky is the limit with gears, as long as you have deep pockets If you do have the 3.07's, I think that carrier is case-specific, so you may be limited to those gears unless you buy whole axles to replace what you have.

c)in a d-250, 3.07's might work good on the strip with a cranked pump and a tighter converter...in a heavier w-250, 3.54's may have the advantage to get the weight moving. jmo

greg

 

i belive the carrier break is 456 and down, anything over 456 gearing will need a new carrier not a whole new axel asembly.

but anything outside of 307 gearing will need a carrier as well i think.

jiMMy

 

I do have the 727 3-speed thing. (I'm pretty sure it's a 3 speed, anyhow... )

So you don't think there will be any benefit to be had from going any lower. That sounds just right to me... Money and time better spent elsewhere.


Thanks.

 

From my limited drag race knowledge, final gear ratio, including tire size, is generally tailored to the length track you are running...a setup for 8th mile will most likely run out of rpm if applied to a 1/4 mile track. So, for example, 4.10's for the 8th, while maybe 3.54's for the 1/4. Just an example, of course.

imho, the 3.07 is a good compromise for the 3sp auto...that's why dodge put it there. I believe your $$ will be best spent on tranny mods--torque converter and valve body come to mind. That'll get the available power to the ground far more efficiantly. Then get pyro and tranny gauges and CRANK THAT FUEL SCREW The dopey grin your face gets is free of charge and standard equipment on all bombed cummins powered trucks!

g

 

I hear you , but I need to get a pyro and tachometer before I do anything, followed rather closely by a better exhaust...


I can imagine that 3.07 is a good compromise, but based upon my maximum speed of 80 MPH I figured that the gear was 4-ish... Guess I just need the 4200 spring.


I was mainly wondering if there was anything smaller I could put in, but it doesn't look like it. So, money better spent elsewhere.

 

yeah, if you had 4.10's, your max governed speed would be like 61

 

These things don't have speed limiters, do they?

Just the fuel spring, correct?
I can imagine that the 3200 would make it go at least 90? What is in there stock?

 

Stock is 2500 RPM, but they will run a bit higher. You ought to be able to run 100 mph with a 3200 RPM spring. If you are going to use that top speed, you need the 60 lb valvesprings to make sure you don't float the valves. The 3200 RPM spring will let the engine run up near 4000 RPM with no load, just pushing the truck on flat ground I'm sure it would rev over 3500. Not that I'm suggesting you break any traffic laws...

 

lots of luck finding 3.07s been discontinued for several years. would like to have a spare set myself. pump screw turned a bit it will pull them nicely.

 

an aux tranny might be a better option, you could leave it in hi all the time...

 

<Knock on wood> I don't think I'll need a spare differential... I've never had a differential go bad on me anyways...


So if a 3200 lets you run around 4000, does a 4200 redline the engine?

Where's the redline on these things anyways? I think the pump is supposed yo be high-RPM inclined, but where does the engine max out at?



As for the AUX tranny, I would love one... Boy would I... But I'd need to rob a bank first...

 

I read somewhere that the timing gear set hits a destructive resonance at 4200...if held there long enough, the gears could shatter, kind of like cracking a crystal glass with a high note. these harmonics can be dampened as obviously floks run these engines higher than that. Question is, why would you want to? Folks are making huge #'s below 3500...
g

 

if you keep the rpms up in the 3500-4200 range dont expect real long engine life the 5.9 is tough but all that weight flying around real fast for long periods will take a toll dont expect 400k spinning it up to 3 or 4 k . I personaly would go to an overdrive...better MPGs less noise easyer on the motor. my .02

 

Hi, Here's something you can play with to calculate RPM/MPH, among other things, http://www.izook.com/tech/math/gearcalc.htm. There are others on the internet. I think the peak torque for my ('89) engine is somewhere in the mid to higher teens. With The Getrag 5 spd, 3.54 diff, and stock tires I run about 2,000 to 2,100 at 70mph, which I think is about right for all around use. I think I read somewhere that the governed speed for my engine was around 2,900, but I've never been there. Because there really are a lot of heavy parts in these engines, I too would believe engine life expectancy would become increasingly shorter as RPMs increased above the max designed speed. Just food for thought. Good luck. Steve