Gear Vendors Over/Underdrive for 68RFE

[PLanciers]

Your kidding, right??? A 3% difference is not even worth looking at. Less heat? C'mon! It doesn't matter what combination you make it will generate the same amount of heat, just in different places. The trans coolers are pretty good on these trucks. With the TC locked you will not be able to see a difference on a gauge.



Let me put this in perspective, $3700 to get ONE AND ONLY ONE split that is any good. Thats a no brainer considering the TQ the engine has.



The rpm's for 70 and 75 are ridiculous. :-laf I seriously doubt the trans will allow that unless its downhill or under coast. From what I hear the 68RFE doesn't like to hold 6th under very much throttle and/or load. The trans will be downshifting to 5th every time the throttle is bumped, if it even manages to upshift. THATS way worse than a little rpm.



There is just nothing in economy to be gained running under 1600 rpms. Its the back side of the TQ curve and with the emissions design its severely defueled. I seriously doubt the TQ peak is that low anyway as emissions plays a large part of the fueling at low rpm's. I'll withhold judgment until I see a dyno sheet but I would guess it will show a TQ peak around 2200 rpms and HP peak around 28000 roms. All thats gonna happen is it will coke the injectors faster and cause more issues.



All I see is a big expensive toy that does little or nothing to enhance what looks like a decent gear split from the factory, and, possibly lead to other problems by letting the motor run under the high idle limit.

I think some are getting wrapped around the axle here, good pun :-laf without looking at my primary issue and I'm sure I'd miss it too if I was sitting on a 3. 54 gear set...



1. I have ordered a 4. 10 gear set to maximize tow capacity...



2. I desire to maximize empty fuel economy and while somewhat ashamed I have been known to liberally interpret highway speed limits from time to time.



3. I agree that the stock gear splits look pretty damn good... and may find that even with the 4. 10 final ratio things are copasetic.



4. If not I'm primarily looking at this solution for final drive relief any additional positives that may be derived is icing on the cake.



I've been quoted un laden highway 22-28 MPG figures by locals here that have such a GV unit, while none of these folk have the 6. 7 68RFE combination, most own second and third generation 5. 9L w/automatics and haul their horses considerable distances... . I'm perfectly willing to listen to and by all means invite alternate more cost effective solutions that are slightly more sophisticated than never drive over 70mph.



Pete

 

[cerberusiam]

I've been quoted un laden highway 22-28 MPG figures by locals

On a 2nd gen 160-180 HP truck, sure I'd buy it. Very doubtful you will see that out of 3rd gen emissions motor. Kinda the nature of the beast. Even Cummins doesn't think they will mileage better than a 2nd gen. :-laf



As I said, I think it is an expensive toy with minimal use that may very well cause other issues, but, Hey! its your truck and your dollar. Whatever you decide, enjoy the truck.

 

[CBorzoni]

I have a Gv unit on my Suburban, NV5600,1st gen 12valve cummins. I like the 6th double od, i run 3. 73 gears and at 70mph in 6th gear it tachs a little high and seems to suck a little more fuel. When i hit the double od rpm's drop and i can run 78-80 mph and the motor seems to be running super low rpms. I get about 1-2 mpg better engaged in double od than running in regular 6th gear. When i tow my boat i run 5th gear od which is about the same gearing it feels as 6th od. I read in the tdr magazine a writer had a nv5600 and installed a GV unit he saw a 100 degree + drop in trans temps when towing his 5th wheel running in 5th gear od compared to runnning in 6th od. For what it's worth i really like my gearvendor unit i like the double od it offers and would not like driving my sub w/out it. I do not tow very much with it but like having the option to split gears. i did not buy mine new so cost was super cheap for me:-laf For the people who don't like the idea great don't buy it, but for the people who are interested in buying a unit it has been a great addition to my rig and has proved to be useful and fun to use, not to mention the increase in mileage.



Later chrisOo.

 

[EMD-Run8]

I dunno, but a . 625 OD (6th) is plenty tall. Personally, I think I'd gone with 3. 73s and left it alone. Or maybe 4. 10s and a little taller tire. Sure, Class 8 trucks use 9,10,13 and even 18 speed trannys-------but they gross like 7 or so times the weight that a truck like a 2500/3500 will (or should) be grossing at. I could see a GV type setup with an NV5600 or 4500 but these new autos are lookin' pretty sweet on their own. The other thing you have to consider is wind resistance/frontal area of your truck if you plan on runnin' empty at 85 plus. Once you start fightin' drag on the vehicle, an OD unit certainly ain't gonna be helpin' I would think.

 

[CContreras]

In addition to fuel savings, there's less wear and tear on the engine and longer oil change intervals due to the engine turning less revolutions.

 

[PLanciers]

Hey EMD-Run8

I dunno, but a . 625 OD (6th) is plenty tall. Personally, I think I'd gone with 3. 73s and left it alone. Or maybe 4. 10s and a little taller tire. Sure, Class 8 trucks use 9,10,13 and even 18 speed trannys-------but they gross like 7 or so times the weight that a truck like a 2500/3500 will (or should) be grossing at. I could see a GV type setup with an NV5600 or 4500 but these new autos are lookin' pretty sweet on their own. The other thing you have to consider is wind resistance/frontal area of your truck if you plan on runnin' empty at 85 plus. Once you start fightin' drag on the vehicle, an OD unit certainly ain't gonna be helpin' I would think.

All good thoughts I had originally thought the answer might be just that 19. 5 inch wheels with 34" inch tires effectively making the 4. 10 approx 3. 74... but then were right back where we started from losing the extra 2K towing capacity, right? Too bad we can't use tires that grow as centrifigal forces increase like Top Fuel dragsters , now that would be slick... parden the pun.



There are real good answers for unloaded fuel economy and good answers for getting the grunt when required... but the only answer I see that may scratch both itches is the overdrive w/4. 10, obviously with the 68RFE I never invisioned a scenario where you'd run through all the ratios, In fact the only split I'm confident will yield value is the 0. 78 with the transmission holding 4th while towing. I'd like to find an answer that dosen't seem like overkill which the GV may be appoaching, it just hasen't occured to me yet. Understand at this point it's just an acedemic exercise to keep me occupied until my truck gets delivered, once I gain some experience with it I may come to realize that it may not be necessary. We really don't have a lot of real data yet on the fuel ecomony aspects of this new combo, plenty of assumptions but not a lot of experience as yet. We'll just have to see what the truth is and factor that in to evaluate the potential here. Also I understand what your saying and the implications of negating any GV mechanical advantages running up against the aero wall.



Hey just noticed you SIG, you a RR man, I was a freight conductor in my youth on the B&A railroad between Selkirk, NY and Boston Mass back before ConRail when it was the Penn Railroad, you remember when we had cabooses and rear brakeman?



Pete

 

[jcarey]

There are real good answers for unloaded fuel economy and good answers for getting the grunt when required...



I don't see why a set of injectors or a box wouldn't be both an effective and an economical answer for the low end grunt as well as picking up some mpg improvement.



I think it's hard tp get the best of both worlds without some software assist.

 

[mberry]

All good thoughts I had originally thought the answer might be just that 19. 5 inch wheels with 34" inch tires effectively making the 4. 10 approx 3. 74... but then were right back where we started from losing the extra 2K towing capacity, right?

You wouldn't be losing the "real" towing capacity of the truck, because the "legal" towing capacity would remain the same. We have all seen some of these trucks run well over their rated capacity without any problems in the short term. If you are using your truck to run on the highway most of the time, I personally think you are heading the right way with the GV unit. I would stick with the 4. 10 for legal reasons if I were in your shoes. Just my opinion.

 

[cerberusiam]

but the only answer I see that may scratch both itches is the overdrive w/4. 10, obviously with the 68RFE I never invisioned a scenario where you'd run through all the ratios, In fact the only split I'm confident will yield value is the 0. 78 with the transmission holding 4th while towing

I am curious why you think this would be worth while. This is very little difference from 5th at . 82, and actually a higher ratio which will increase the load. From the looks of the trans design you are engaging the same amount of planetaries as drive to achieve these ratios. I can not see any appreciable gains, certainly not enough to justify $3700, by going this route. Loaded to the max GCVW it would seem to be a step backwards in minimizing stress on the drive train.

 

[PLanciers]

I am curious why you think this would be worth while. This is very little difference from 5th at . 82, and actually a higher ratio which will increase the load. From the looks of the trans design you are engaging the same amount of planetaries as drive to achieve these ratios. I can not see any appreciable gains, certainly not enough to justify $3700, by going this route. Loaded to the max GCVW it would seem to be a step backwards in minimizing stress on the drive train.

Towing in O/D creates excessive heat, 5th & 6th are O/D ratios in the 68RFE...



Members from the RV/towing forum are telling me they're experiencing trans temp deltas of 100 degrees or more less by towing in a non overdriven ratio with their GV or US Gear units in overdrive. Any thing that drives down transmission temps while towing is a big upcheck in longevity. Which probably brings us to my next post which will probably be "What have you done to significantly improve driveline friction, and when should it be accomplished in a new trucks break in cycle?"



I'm sure someone has and will spend $3700. 00 on one of these units, that's not my intention. <



Pete

 

[cerberusiam]

Towing in O/D creates excessive heat, 5th & 6th are O/D ratios in the 68RFE...



Members from the RV/towing forum are telling me they're experiencing trans temp deltas of 100 degrees or more less by towing in a non overdriven ratio with their GV or US Gear units in overdrive. Any thing that drives down transmission temps while towing is a big upcheck in longevity. Which probably brings us to my next post which will probably be "What have you done to significantly improve driveline friction, and when should it be accomplished in a new trucks break in cycle?"



I'm sure someone has and will spend $3700. 00 on one of these units, that's not my intention. <



Pete

Ah, I see what where you are coming from now. As for the 100 degree differences you need to ask with what, how much, and when. That number is hard to believe considering what I have seen out of the 48RE so far. I have been towing 10k+ several times over the last month and have yet to break 200 degrees. Most of the time at cruise it is around 150 loaded. Granted ti sin't very warm yet so will withhold final judgement until I have done some heavy towing in the heat.



As for 5th and 6th being OD, take a look at the design of a 3 pinion trans compared to the 47/48RE. There is a big difference in how the OD is achieved and the pieces that are achieving it. If, and I say if becuase there is not data on the new trans yet, the 68RFE is built as described there should not be that kind of problem. The AISIN is not seeing this issue that has been reported so far and the 2 are fairly similar in design.



The preliminary designs I have seen so far look good for power transmission. How they hold up is yet to be seen. Cheers.

 

[cksh8me]

Let's say that the splits with the GV unit fell right in the middle and all was well with the world.



GV says you shouldn't use their unit with E brake.



If I'm pulling a hill in something over drive get to the top and now going down the hill I have to shift up or down to use the e brake?



If I have to have a gear in the middle to keep me in the power then why don't I need a gear in the middle to keep me going down hill at the right speed?



What if you don't use your E brake can you go down in compression?