how to lower RPM

[ilovetrains]

Saw a post about the G56 and final drive ratio, can't find the dang thing now.



Who (if anyone) makes gears for these? Want to drop down to 3. 00 or close to it.



The alternative seems to be adding an overdrive. Which is the better option?



I run about 2600 RPM at 79-80 MPH which is too darn high. Want to get this down to about 2000 RPM, then start bombs.

 

[67guzzler]

Trains -- This thread? https://www.turbodieselregister.com/forums/showthread.php?t=135883 I gave up. I even called a ring and pinion shop and they said that the lowest set of gears for our differentials is 3. 73 so there's no place to go. Then I checked with US Gear and Gear Vendors for overdrive units. I couldn't stomach the $3,600 installed for the Gear Vendors unit. Then I checked on 19. 5" wheels and 35" tires. That would get my final drive ratio down to around . 71 +/- from . 79 and reduce my 70 mph rpm's from 2210 to around 1980. But that would also cost $3,200 from Ricksons w/Hancook tires. I finally ended up just going with the 2nd cheapest option of all -- getting big fat cheap Cooper Tires with a 34. 85" diameter. Close to the Rickson combo, 2" wider and $2,400 cheaper..... Naturally the cheapest option of all is to do nothing... . Sorry for the bad news.....

 

[TimG]

The best way I've found to lower RPM's is to insert a brick under the "Go Pedal". LOL You can buy a lot of fuel for the price of some of these fixes.

 

[ilovetrains]

wow - that sucks.

Unfortunately I cannot go bigger on the tires, this thing fits in my garage by about 1/2 inch as it is.



I may have to go the GV route. Yeah - that is a lot of diesel but I spending about 150 a week right now. It adds up much quicker than you think.

 

[bpenrod]

My GV didn;t help the fule mileage much, maybe 1 mpg. I have a 200+ mile daily commute. I used to do it at 70'ish because I was spinning over 2100 rpm. 2000 seems to be where mileage drops off. Now with the GV installed I cruise at 78 to 80 and 1800 rpm at about the same fuel mileage as doing 70. If I keep it at the same speed as before I see an increase in economy but 78 to 80 seems really comfortable cruising down the road. Even with small gains in mileage I would do it again just for the reduced cab noise levels, nice and quiet at 1800 rpm!!

 

[67guzzler]

My GV didn;t help the fule mileage much, maybe 1 mpg. I have a 200+ mile daily commute. I used to do it at 70'ish because I was spinning over 2100 rpm. 2000 seems to be where mileage drops off. Now with the GV installed I cruise at 78 to 80 and 1800 rpm at about the same fuel mileage as doing 70. If I keep it at the same speed as before I see an increase in economy but 78 to 80 seems really comfortable cruising down the road. Even with small gains in mileage I would do it again just for the reduced cab noise levels, nice and quiet at 1800 rpm!!

The Aerodynamics are a killer much over 55 mph. I think you're having a huge fuel savings if you're going 78 to 80 mph and saving 1 mpg over going 70. Check out this chart:



#ad




Note the curve and the power requirements between 70 mph and 80 mph. If you're saving 1 mpg and you're going 80 vs. 70 you're saving fuel big time!

 

[bpenrod]

Nice chart, makes sense to what I have seen as well. I was getting around 19 to 20 running 70 or so before and now I'm averaging 20 to 21 at 78 or so.

 

[67guzzler]

I got the chart from the Cummins PDF that someone linked to here in another thread regarding fuel economy. Going 78 to 80 and getting 21 is very impressive.

 

[LesStallings]

I run about 2600 RPM at 79-80 MPH which is too darn high. Want to get this down to about 2000 RPM, then start bombs.

Dropping the RPM and starting bombs seems contradictory to me. What are you trying to achieve?? A really quiet diesel at highway speed or one that will pull your house over to the neighbors yard? Do you want a hot rod diesel?



I ask this because I initially thought my truck needed a 7th gear to reduce the RPM on the highway. After a while, I realized that it would be nice to have 4. 10's to keep the engine in the sweet spot of 2,200 RPM for maximum towing power. I call that a pretty radical turnabout in what my desires were.



I suggest getting used to your diesel before you change things. There are a lot of used parts for sale that at one time were the desires of the owner but turned out to be the wrong thing.



Just a thought. Enjoy your truck!

 

[bpenrod]

With the 6 speed can you keep it in 5th (direct) and stay at 2200 with the 3. 73's? One nice thing about the hand-shaker, it's a little easier to find the right gear for almost every need.

 

[67guzzler]

For me the point is that I'd like the overdrive to be an overdrive. I'd like 5th to be my towing gear. It would have been nice if they had actually geared this thing so that you could tow in 5th and keep up to say 60 to 65 at 2200 rpm's with a 1 to 1 ratio in the transmission. Then 6th would be a true overdrive that you could use when you're not towing. It would have been nice... . I think that supply limitations got in the way on this one and DC went with the G56 out of necessity.

 

[bpenrod]

Thats another good point for the GV (or any) OD unit. With the Dodge trans in 4th (direct for my 5 speed) and the GV in OD (4th Over) 2100 rpm equals 66 mph with the factory 265 tires. I use that alot when towing larger loads.

 

[ilovetrains]

at 3600 the GV unit is awful high. Does anyone make a overdrive unit that is cheaper. Also, someone has to make 3. 55 or lesser gears for these things, they used to come with that ratio, did they change both axles?

 

[67guzzler]

They changed the axles. Spoke with two ring and pinon specialists and they said after looking it up to confirm that they start with the 3. 73 and you can go from there to the 4. 10 and the 4. 56... .



Who knows? Maybe they're wrong?

 

[DPKetchum]

Differant axel manufacture. Not available in a AAM axel. Dana had 3. 55s. They sucked for towing. D. C. wanted away from the 3. 55s back in 2000. Said 3. 73 was a good compromise gear for the Cummin.

 

[67guzzler]

Differant axel manufacture. Not available in a AAM axel. Dana had 3. 55s. They sucked for towing. D. C. wanted away from the 3. 55s back in 2000. Said 3. 73 was a good compromise gear for the Cummin.

That's true with a final 48RE ratio of . 69 or the NV5600 ratio of . 73. But this G56 is at . 79 -- a 3. 55 sure would be nice about now. Maybe a 3. 2x even.



After considering all the options I'm getting 315 tires this week so I'm over it. Effectively my . 79 is down to under . 73 and my 2200 rpm @ 70 mph cruise will be at around 2010 rpm's. After considering all the options, it was the cheapest solution I could come up with. I really love the truck otherwise.

 

[Vaughn MacKenzie]

I have always thought the best setup would be approx 3. 2x rear end ratio with stock tires on a 4x2 truck and 3. 3x on 4x4. That way you can have the right towing RPM & vehicle speed in 5th gear around 2100-2300, and use 6th for good cruising speeds around 1900-2000 RPM when empty.

 

[hemijustin]

i had a GV overdrive, but it blew up and left me on the side of the road with my trailer... (no i wasent towing with it on... ) and i called up GV to see about getting it fixed and they said its common for stick shift dodges to break them, so i just left it off. i miss it on long trips with the 4. 10 gears im runing alot of rpms to cruse at 70mph but oh well. i just got biger tires

 

[bpenrod]

WOW, thats the first I heard of a GV blowing up. Did yo use the clutch when shifting in and out of overdrive? So far mine is doing good, I changed the fluid when i did my clutch and it looked god. I don't imagine it gave you much warning when it blew, did it? I kept my original tail shaft and drive shaft so I could switch back easily if I need to. If it breaks that willbe my back up plan.



On another note, took a road trip to Nashville this week (still here) and the truck did great. Filled up North of Findlay Ohio and drove to Nashville, around 470 miles. I have since drove 118 miles around Nashville and when I filled up today my overall mileage was exactly 21. I should have topped off after the highway part to see what it was. !!

 

[67guzzler]

Emails to AAM. Waiting for a reply.

Guys -- below is a email thread that I started with AAM this morning.



They seem like nice people as they replied within hours.



I don't know if is going to get anywhere but I figured it can't hurt.



**********************************





> -----Original Message-----

> From: Jay Gann

> Sent: Tuesday, August 23, 2005 9:08 AM

> Subject: Dodge Ram with AAM Rear Axle

>

> I have a 2005 Dodge Ram Cummins with a 11. 50 AAM rear axle.

>

> Do you have any ring and pinion sets with ratios taller than 3. 73? I can get 4. 10 and 4. 56 but I'd like to be able to get a 3. 54 or a 3. 23 or a 3. 08 for my truck.

>

> If you don't make anything taller than a 3. 73 such as the 3. 54, 3. 23 or 3. 08 who would I provide feedback to making this suggestion?

>

> Thanks.

>



Laughman, William wrote:



> Jay,

>

> Sorry but we only provide the 3. 73 and 4. 10.

>

>

> Bill Laughman

> American Axle and Manufacturing

> Aftermarket Account Manager

>

>** Scanned by AAM SymantecMailSecurity XXXNT66 **

>



Bill -- Thanks for the reply.



The reason why I asked is that DiamlerChrysler, in their infinite wisdom in 2005 has changed the 6 speed manual to a Mercedes Benz G56 transmission from the NV5600 they had before. This transmission has a final drive ratio of . 79 in overdrive whereas the NV5600 was at . 73 in overdrive and the current 48RE automatic is at . 69 in overdrive. The net-effect of this is the 3. 73 rear is effectively a 4. 10 with the new G56 6 speed. So at 70 mph, I'm turning 2200 RPM on the Cummins when I'd like to be around 1800 to 2000 max with this engine.



DiamlerChrysler knew they screwed up with this combo so much so that if you get the 6 speed you can't even get the 4. 10 because they know it's so low. So the point of my contacting you was to let you know that your customer (DiamlerChrysler) may have a need that they don't even realize yet (and a bunch of unhappy customers who didn't realize the effectively low final drive ratio they're getting with the new G56 manual transmission). It's the talk of all the diesel boards. Thanks for your time and the reply. I hope my comments are of some value and might result in you all considering a taller ratio sometime down the road.



Regards, Jay Gann