Changing differentials vs. aux overdrive

[RWheeler]

Just got my 04 2500 4x4 CTD. LOVE IT. Only thing is I had to settle for a 4. 10 diff with the six speed transmission... About 85% hwy miles >70 MPH at 200 miles a trip. I put the 315 tires on it and I'm turning 2350 RPM's at 70. Thinking about how to get the RPM's down at hi speeds. Should I go with an aux overdrive or try to change the diff to 3. 73 or lower?

 

[ErniePink]

RWheeler,



Had same concerns with my '96 3500 4x4 5 speed. 1) Ya could lighten up on the go pedal... ... ... . nah!! 2) I went to Gear vendors after doing too heavy research; will cost ya about $2600+ installed. I really liked this unit but had several repair problems over the 5 years I used it. GV replaced the unit and install costs once during first 12 months. Very pleased with their assistance from anywhere in USA. I was a long way from home and was able to use all direct gears to get home. BTW, I couldn't use 5th over towing my 5er RV unless I was streaking in excess of 80 mph. Waaaaay tooo fast hooked up, I say. The split between 4th and 5th was a great puller and handled lots of upgrades that 5th wouldn't pull without lugging, hooked up of course. With the GV you'll have 9-10 gears fwd. Lots of fun but some confusing for my older confuser brain now. Ex brake worked great with the GV. Ya won't be able to use the GV when 4 wheeling. 3) If ya go to the 3. 73's will DC redo your warranty? 4) Consider the diff in fuel mileage between the two ratio's and do the math to see if it's cost effective for your needs. 5) Learn the truck for awhile and then Bomb it for more fun; won't help the r's but will be more fun to drive and usually will help with the mpg numbers.



Hope this helps ya some.

 

[Moterhead]

Originally posted by RWheeler

Just got my 04 2500 4x4 CTD. LOVE IT. Only thing is I had to settle for a 4. 10 diff with the six speed transmission... About 85% hwy miles >70 MPH at 200 miles a trip. I put the 315 tires on it and I'm turning 2350 RPM's at 70. Thinking about how to get the RPM's down at hi speeds. Should I go with an aux overdrive or try to change the diff to 3. 73 or lower?

Lots of list members in Texas. Perhaps someone with 3. 73s wishes they'd gotten the 4. 10s. Just swap 'em out.

 

[bulabula]

Rwheeler, both options that you listed are high cost. It is a good way to reduce revs on the superslab slightly, but if your intention is ultimately to improve fuel mileage, figuring the cost to be a couple grand just to realize a couple mpg improvement, you'd have to drive LOTS and LOTS AND LOTS of miles just to break even on your investment - do the math. And if the guy who did the R&P changeout wasn't that good, you'll also have to live with excessive gear noise as well. I'd try to live with it before spending those kinds of bucks.



Now if you can find a fellow that wants to trade diff's one for one, that'd make it worth it.

 

[klenger]

I had 4. 10's in my 01. 5 and it worked OK for me. Cheapest thing you can do is just drive it and put fuel in it when it gets low. As a point of interest, check the total engine time using your odometer, and calculage your average MPH over the current lifetime of the truck. I think you will see that you aren't driving at highway speeds as much as you think.



I personally do like the idea of another transmission (under/overdrive). I've heard stories of 50,000 mile life span, can't use it with an exhaust brake, no improvement in milage, etc. Some members have them and love them. Bless them.

 

[RWheeler]

Thanks for the advice fellas,

The problem is that I run back and forth from Corpus Christi to Austin twice a week (400 miles roundtrip). Most of that is interstate mileage so the benefits that can be recognized through gearing seem to add up at about 3. 5 - 4 years to payout the investment in fuel alone, not to mention the gains in engine and component wear and tear. So I think the best idea is to find someone out there who wants to swap... anybody... anybody. But I am planning on putting about 25k on her before the bombing commences to really notice the gains.

 

[Dkevdog]

Originally posted by ERNIE PINK



Ex brake worked great with the GV.

That is not true- you will destroy the GV unit if you use it with an exhaust brake. GV unit must be disengaged (direct) in order to use the exhaust brake.



The US Gear overdrive is the only one that allows for the use of the exhaust brake either in over or direct.



I would love a Gear Vendors, but the $3K price tag tends to hold ya back some. Cost of a gear swap (at least to what I've been told) is around $1000- this is a 2wd. Still fairly pricey.



Kev

 

[ErniePink]

Dkevdog, Rwheeler and others. ... .



Confused am I. Sorry to confuse others, no Ex Brake use with Gear Vendors engaged. Mea culpa many times... ...

 

[Dkevdog]

Couple of times with the E-brake probably ok, but I know for fact it will eventually destroy the unit. I know of two such incidents first hand, and a quick call to GV, (which I did) confirms it. The clutch assembly can not handle the reverse torque an E-brake places on it. The Gear vendors unit is a clutch unit, similar to an automatic trans. The confusion might come from the fact that you can have the GV unit on truck with an Ebrake- when you engage the Ebrake, the GV unit automatically disengages and drops you into direct (When properly wired to the ebrake).



The US Gear unit however, does NOT have clutch assmeblies. It works very much like a manual transmission- changing the driveshaft rotation ratio. It is directly coupled, either in or out, so the Ebrake will not cause any problems, regardless of if it is in direct or OD.



On that note however, this is why the US gear model can not be shifted under throttle, like the GV unit can. It is also "clunky" when it fiirst engages and I did not like that (in the test truck I drove). Gear Vendors claims that the shock is within the unit and not the driveline, but I didn't care for it. Please know that I do not OWN either product, but have driven (test drive) both. I did alot of asking and searching, so I am somewhat educated on them.



IF only it wasn't so much $$.



Kev

 

[DPKetchum]

Why not some 19. 5s tire and wheel combo. With thart uch driving longer tire life and reduced rpm's?

 

[Peter Campbell]

I also have the 4. 10 /6 speed. Thinking about the 19. 5's Got that setup for the nameplate tow rating. Did you recalibrate the speedo-with the stock tires it is around 2300 at 70 MPH ( 657 revolutions per mile ) if it is 10 % bigger tire ( 3" dia ) then you should be 2030 at 70. Or now you are doing 77 MPH indicating 70. Slow down to exactly 60 MPH indicated put it on cruise and clock one mile- that will give you a rough idea of the speedo error if any.

 

[Blakers]

Go with some 40" tires. That would really look good too.

 

[jcarey]

Someone correct me if I wrong (math is not of of my strong points), but I'm not sure a GV would help at crusing speed. GV's final gear ratio is . 78 to 1, whereas the NV5600 trans has a . 73 to1 final gear.

 

[klenger]

jcarey



The aux transmission is not in place of the the NV 5600, but in series with it. ,78 * . 73 = . 57, or looking at it another way, it turnes a 4. 10 axle into a 3. 19. (Wow, that's low, or high depending on how you look at it).

 

[jcarey]

Thanks. That's good to know. I've been looking at the GV, myself.

 

[abledog]

Sounds like Peter has hit the nail on the head! If you have a GPS use it to check your speed.

Terry

 

[XJSuperman]

Jump to 37" tires OR swap axles with someone who wants 4. 10's for their 3. 73's.

 

[1rumrunner]

Originally posted by RWheeler

Just got my 04 2500 4x4 CTD. LOVE IT. Only thing is I had to settle for a 4. 10 diff with the six speed transmission... About 85% hwy miles >70 MPH at 200 miles a trip. I put the 315 tires on it and I'm turning 2350 RPM's at 70. Thinking about how to get the RPM's down at hi speeds. Should I go with an aux overdrive or try to change the diff to 3. 73 or lower?

RWheeler,



I've got a '03 2500 Quad SWB with 3:54s and I put on the Gear Vendors unit. I think its great because I do a lot of driving back and forth from LA CA to Lake Havasu AZ (600mi RT) and the the truck gets GREAT mileage. If I had to do it again, I would get the 3:73 gears W/ GV unit because I can't engage the unit until 65mph+. At 80MPH my RPM is 1700!



Gregg

 

[Mopar_Mudder]

I think that the overdrive is really a waste for gaining milage also.



Think about it, you are going to spend close to $3000 to put it on. With that if you gain an average of 2mpg it will take 250,000-300,000 miles to pay for its self. And face it, most of people don't keep a vehicle that long, and will and overdrive even take that many miles?



If you can't swap for cheap with some one that has 3. 73's I would forget it and just drive. The RPM's aren't going to hurt the Cummins, this same basic motor used in marine aplications may run for days at a time full rpm.

 

[Spooled-up]

I agree with Peter Campbell.



It seems that your rpm's would be much lower with that size tire. I have the 4:10's with the 6-spd and at 70mph, I'm doing around 2350rpm's with the stock 265 tires. If you didn't get the speedo recalibrated, the speed/rpm ratio's are going to remain the same.



Without the recalibration, you're going faster than your speedo says. I'd get the recalibration and then check your speed against your rpm and see how you like it.