Overdrive or Taller Gears for Fuel Savings

[Bernie]

I have an 06, HO, 6 speed manual 4x4 with 3. 73s. The truck needs another gear at highway speeds! I'm turning about 2400 RPMs just to keep up with traffic at 75 MPH. If I'm late and I need to move, it's even worse.



Please, save the e-mails and posts telling me to slow down. The safest speed is with the traffic around you. If that is 65 MPH, I go 65 MPH. If that is 85 MPH, I do 85. If it's an empty highway, I do about 10 over the posted limited (maybe a little more if I'm late).



I drive about 45k miles a year, mostly highway, so if fuel is $3 a gallon a 3 MPG savings (from 16 to 19) means about $1300 a year.



I have found this overdrive:

http://www.drivetrain.com/new_overdrive/Dodgemanual/overdrivedodgmanual.html



I figure (with shipping, handling, shortening the drive shaft, and $400 for an install) this unit will cost me about $3700. It will take me three years to pay it off (in fuel savings).



The problem is:

-I want to wait until my warranty is over to modify the truck (2 years)

-Three more years to pay for the unit (truck would then be 5 years old and have 225k miles on it)

-If I keep the truck until 300k miles I've only saved about $2200 in fuel total.



My questions are:

-Has anyone used this overdrive? What do you think? What type of milage do you see?

-Is the final drive of 2. 13 (with my trans and axels) possibly too tall?

-Would it be easier/less expensive to change my diffs out to a taller ratio? Is there a 3. 53 (front and rear)for my truck? I belive I have the 11. 5" rear end.

-Any other thoughts?

 

[DieselFiend]

Are you sure you've got 3:73's? Don't mean to insult you or anything. You'd obviously know better than I would but I've got 3:73's with my auto and I turn a little over 2000 RPM at 75 and about 2200 at 80. My buddy had a manual, same year, and he was turning just a little less than me at the same speed. Now at the time we tested, he had 315's and I had my current set-up which is 305's on 20" rims. I know that makes the ratio a little more favorable. Sounds like something isn't right. Don't know what though.

 

[DGStrate]

Are you sure you've got 3:73's? Don't mean to insult you or anything. You'd obviously know better than I would but I've got 3:73's with my auto and I turn a little over 2000 RPM at 75 and about 2200 at 80. My buddy had a manual, same year, and he was turning just a little less than me at the same speed. Now at the time we tested, he had 315's and I had my current set-up which is 305's on 20" rims. I know that makes the ratio a little more favorable. Sounds like something isn't right. Don't know what though.

Remember though that the 48-RE Transmission has a . 69 OD ratio and the NV5600 had (I believe) a . 71 or . 73 OD ratio.



The new 6-speed (G56) has a . 79 overdrive ratio, which raises the RPMs significantly. This is one reason the G56 is only offered with the 3. 73:1 rear end gears. Having a G56 + 3. 73s is like having the NV5600 + 4:10s due to the overdrive difference.



Heck - there are times (driving empty) that I would like the OD unit OR 3:5X gears with my 48-RE (we tend to drive either stop-and-go or B@LLS OUT FAST in MI!)...

 

[Bernie]

I double checked

The gearing seemed low to me as well.



-I never got to test drive a 6 speed manual, only an auto

-I figured the manual (being 6 speed instead of 4) would have a taller top gear ratio (more gears).

-When the truck arrived I noticed it was lower geared than I thought it would be.

-My first thought was it was 4. 10 gears. My build sheet lists 3. 73s. I double checked with my dealer, another dealer, and on this forum. All verify that my build code and vin number come back at 3. 73 gears.

-Per the various websites I have found with the transmission ratios listed, the final drive of my stock transmission and axles combination is 2. 72

-The same sources list the 4 speed auto as having a taller 2. 57 final drive with the same axles.

-I may have been off with my estimations of the RPMs I have. I know that to stay below 2000 RPMs (where I seem to get good economy) I need to go below the 65 MPH speed limit!

-I will double check this afternoon when I have to go see a customer and let you know the exact RPM at 75 MPH.

 

[Bernie]

Here is what I found

On the gearing for my truck:



NV-5600 Six-speed overdrive:

This transmission is standard with the high-output Cummins.



Gear Ratios:



1st: 5. 63

2nd: 3. 38

3rd: 2. 04

4th: 1. 39

5th: 1. 00

6th: 0. 73

Reverse: 5. 63

Overall Top Gear Ratio: 2. 72 with 3. 73 axle and 2. 99 with 4. 10 axle

 

[AAston]

NV-5600 or G56

Bernie,



I have an 05 2500 with the G56 transmission. It has a . 79 ratio in 6th. I believe you also have the G56 tyranny since you have an 06. The RPM figures you listed pretty much match mine. Since I don't pull heavy, (8K max about 5% of the time) it took until around 35K miles before my mileage finally improved. On a recent 1200 mile trip running empty between 65 & 70 the whole trip, I averaged 19. 3 MPG hand calculated. That is the best I have ever seen in this truck at that speed.



An overdrive would probably be the better way to go compared to larger tires because of the trade off of increased wind resistance as the height of the truck increases. The difference may be minimal though, as the increase in tire size you can fit without a lift is pretty small. That is why I have held off putting 35-1250-R17s on my truck.



Hope this helps,



Allen

 

[Hercules130]

You have a G56 and there hasn't been an HO since 2004. I don't think the overdrive would ever pay for itself but it sure would be nice to have.

 

[Bernie]

You have a G56 and there hasn't been an HO since 2004. I don't think the overdrive would ever pay for itself but it sure would be nice to have.

Two questions:

-Why do think an OD won't pay for itself? If it increased MPG, there will be a savings. If it saves you $x per year in fuel, and the overdrive costs you $3X, it will pay for itself in three years.

-I thought I had a NV-5600 transmission (6 speed manual). It looks like I was wrong. I need to find more information on the G56.

-Every piece of doccumentation I have from Dodge lists my motor as an HO.

 

[GECCO]

An overdrive would probably be the better way to go compared to larger tires because of the trade off of increased wind resistance as the height of the truck increases. The difference may be minimal though, as the increase in tire size you can fit without a lift is pretty small. That is why I have held off putting 35-1250-R17s on my truck.

Something else to consider is that when you go to a larger tire you will negate some or all of your mileage gain (through lower RPM's) with the higher rolling resistance of a wider tire, and the fact that the tire weighs more. Not only is this weight you have to haul around, but, more importantly, it is weight you have to rotate, and weight you have to rotate through a larger diameter. All of these factors use up more energy. Especially in in-town driving, the larger tire will have a significant negative effect on mileage.



Also, the unit mentioned above has a . 78 ratio (basically a 22% decrease in RPM). My stock tires are 265/70R17's, with an approximate circumference of 99. 3". Going to a 315/70R17 would only net an 8% decrease, a 315/80R17 gets you a little over 14%. To get 22% you would have to go to 345/80R17, if such a tire even exists (Probably does, I have no idea). If you are looking for fuel mileage, this is NOT the tire for you.



The OD unit would be a one-time purchase, where the larger tires would have to be replaced again and again, at a higher cost than the stock size tires.



I have been seriously considering this OD unit, I do a lot of empty highway driving in my truck, when I am towing a trailer it weighs less than 3,000lbs. My only concern would be exhaust temps... . how high would they go at 80mph turning say, 1600rpm?

 

[GECCO]

You have a G56 and there hasn't been an HO since 2004. I don't think the overdrive would ever pay for itself but it sure would be nice to have.

Actually, as I understand it they have ALL been HO since 2004, the non-HO designation is what was dropped.

 

[JasonCzerak]

Two questions:

-Why do think an OD won't pay for itself? If it increased MPG, there will be a savings. If it saves you $x per year in fuel, and the overdrive costs you $3X, it will pay for itself in three years.

-I have a NV-5600 transmission (6 speed manual). Every piece of doccumentation I have from Dodge lists my motor as an HO.

you have the G56. . where is reverse? is is top right (to the right of 5th?)

 

[Ross]

I just bought a new 2006 Mega Cab Dually with the G56 manual. As someone has said, 3. 73 is the only ratio available.



It is pretty darn low geared. Out here in the SW, Interstate 10 has an 80 mph speed limit. My truck is wound up like an 8-day clock. The RPMs are busier than a stripper on dollar night!!! :-laf I too would like another OD, but don't think the expense is worth it. I see lots of rebuilt gear vendors OD's on ebay. They must break alot.

 

[BigEasy]

Bernie, your best bet would be a ring and pinion swap, should be under a grand to do both ends... call Chad's Ring and Pinion in Jamestown or OBrien Transmission (between SugarGrove and Youngsville PA). I have utmost faith in both shops...

 

[WOT]

To my knowledge there isn't anything < 3. 73 for the American Axle (11. 5"). I would love to find a 3. 42 or so...

 

[Bernie]

Bernie, your best bet would be a ring and pinion swap, should be under a grand to do both ends... call Chad's Ring and Pinion in Jamestown or OBrien Transmission (between SugarGrove and Youngsville PA). I have utmost faith in both shops...

Thanks. Is "Chad's Ring and Pinion" is the shop on the way to Lakewood (Route 394 I think)? I've driven past it several times and thought I should stop in there and see what they are capable of. I've changed out differentials in my BMWs, but I move the whole differential as a unit (just attach the half shafts). If you keep them upright you can even leave the fluid in (vent is on the top).



-What is involved in doing the gear swap with my Quaife (non slip) rear end?

-Does anybody know what else will fit in my axels?

 

[Bernie]

G56 Compared to NV5600

I was wrong. I found info online on the NV5600 and per the (incorrect or out of date) website I visited it listed the years as 2002 and up. I thought that was the transmission I have.



So, what is the difference between the G56 and NV5600?

 

[JStrasser]

Bernie,



Jumping in here- I've got an '06 with the six speed. The NV5600 has a . 73 overdrive while the box in our '06's, the G56 has a . 79 OD Less overdrive with the new trans.



I feel the same way about spinning the motor



Quick story- my wife's VW passat that I drive frequently has a 5 speed manual that has 5th gear up and to the right. Several times I have been listening to the news while driving the truck and will find myself shifting out of 6th and moving the shift lever up and to the right to find the next gear. oops. But if you drive a lot of highway miles, it feels like there should be another gear.



Thought about taller tires myself, but much taller would keep me from parking

in the garage at home. Guess I should have bought the auto :{



John

 

[Bernie]

Quick story- my wife's VW passat that I drive frequently has a 5 speed manual that has 5th gear up and to the right. ... will find myself shifting out of 6th and moving the shift lever up and to the right to find the next gear.

Yea, I try to shift in seventh gear quite often. I just did a trip with some friends and all three of us did the same thing several times.

 

[steved]

I was wrong. I found info online on the NV5600 and per the (incorrect or out of date) website I visited it listed the years as 2002 and up. I thought that was the transmission I have.



So, what is the difference between the G56 and NV5600?

Different manufacturers... and the G56 is geared much lower...



I too have been watching for lower gears, I currently run a 5600 with 4. 10s (equal to your G56 with 3. 73s)... and 3. 73s are the lowest numerically offered... so for now, I'll just drive slower...



Also, if you are trying to justify the cost via fuel savings... it usually doesn't work out as you put it on paper...



steved

 

[wsurf]

Back to the original question; to overdrive or not to overdrive, or go bigger tires.



I had a 03 dually with the auto. I put a Gear Vendor on it for towing (gear splitting). It worked great, very nice unit no problems. Biggest advantage (other than towing) was RPM reduction, lower sound level. Reduced RPM by about 500 in OD with the 48RE.



I now have a 06 QC 4X4 LB with the 6 speed and have the same problem as Bernie, high RPM at crusing speeds. I'm considering the Gear Vendor again as (bigger) tires wear out, stress the front end, and are not fuel efficient.



But I would say that the overdrive unit does not save on any fuel. I did not notice any significant fuel savings with the GV unit. I suspect the additional internal friction eats up any fuel savings. But will reduce RPM. There a little pricy so fuel economy payback is years.