Engine/Transmission (1994 - 1998) 6 sp vs. Overdrive

[Barry Fuller]

I was reading the thread about the 6sp conversion from the 5sp and wondered what folks thoughts were about wether that was better than going with the overdrive. Seems like there is not a real big $$ difference, you would get rid of the black hole between 3rd and 4th, but with the 4. 11's I would still be turning pretty high RPM's in high gear.



Bear

 

[C. Harlan]

I used to run 2000@65 in the 4500... now I run 71@2000. . not too bad of a difference. . it is 2% taller than the 4500... Not much in #'s, but does make a difference going down the road. . I can easily go over 100 on the dash now, and still have more pedal to go... . before, it would only get to about 97.



-Chris-

 

[Loren]

Bear,



Go with the auxiliary overdrive transmission. A ten-speed tops a six-speed every day.



I have a six-speed with a U. S. Gear overdrive. I chose U. S. Gear instead of Gear Vendors because I can use it in 4WD. That's not allowed with the Gear Vendors, because it attaches to the rear driveshaft only.



An auxiliary transmission will install much more easily, and you will be happier with the results.



Loren

 

[SMorneau]

I really like the gear splits in the 6 speed. It is the same overdrive ratio in 6th as the 4500 was in 5th. As far as the "10 speed",unless you were pulling a train you wouldn't use the splits in the lower gears.



There are times, on the freeway, when I wish I had another top gear or two. But IMO, the aux transmissions are not strong enough to hold up to a turned up Cummins. I had the six speed transplanted into my truck because I was tired of 5 speed problems so I will not add something else that is going to fail me.



Compare the size of the components in the 5600 with those in the aux transmissions, think about how you're going to BOMB and use your truck, then decide.

 

[Loren]

I would suggest that if you have never had an auxiliary overdrive, you don't know how you would drive with it. I use 3-over, 4-over, and 5-over a lot. I don't do too much freeway driving without the trailer, so I don't have many occasions to use 6-over, but it does come in handy occasionally. Like SMorneau, I also often felt that I needed one more gear above 6th. Now I have it.



As far as the strength of the units is concerned, I can only say that the Gear Vendors is used in drag racing, with much more torque and horsepower than our diesels produce. The U. S. Gear unit weighs about 75 pounds and contains only two speeds. It is almost as big as the NV4500 transmission.



I have heard of only one failure of a G. V. transmission, and no failures of the U. S. Gear unit. If these things couldn't stand the torque, I am sure that it would be all over this web site. So far, my U. S. Gear transmission has handled my engine for about 5000 miles. I have TST injectors and a Power Edge Comp box. About 25% of my mileage is pulling our 9000 pound travel trailer. I suspect that my U. S. Gear transmission is strong enough.



I consider my auxiliary overdrive transmission the second best accessory I've added to my truck. Number one is the exhaust brake.



Since you have the 4. 10 differential ratio, you really could use the higher gear. A six-speed won't get that for you.



If you do it, you won't be sorry.



Loren

 

[SMorneau]

I am in no position to argue about the reliability of the aux transmissions. I will say the 12V engines are hard on transmissions and its not just due to torque or hp. The torsional vibration is the killer. I was told by somebody who engineers transmission parts that about 1,000 ft-lbs was the limit for aux trannies. So if you stay below that you may be fine.



But, no matter how reliable the aux transmission is, you will be mating it to a NV4500 and there are many documented failures plastered all over this site. I have not seen a fix for the synchro failures.



An Extra gear and a marginal transmission or reliability with one less expensive unit to fail? I made my decision.



Good luck no matter how you decide to go.



Loren, I submit that if you have never had your 5 speed replaced or rebuilt 3 times (twice on vacation) you don't know how bad you would want a 6 speed

 

[krs]

A person considering a change to the 6 speed might want to do some reading in the 2nd gen. 24 valve forum. The transmission has been fraught with failures that, although in many cases have been still covered under warranty, would amount to a catastrophic expense once the warranty period has passed.



As to torque ratings - the six speed rating is 550 ft. lbs, only 90 more than the 5 speed.



The auxilliary overdrive units have a long successful track record and can be moved to a new vehicle at trade-in time.

 

[Texas Diesel]

The 6 speed is much stronger than the 5 speed. I love both, my 6 and my wifes 5. The 6 is better for towing and the 5 is much easier in town. Read my profile, I'm on my 3rd 6 speed, this one is doing fine at 33k miles but if it dies on my nickle I'm swapping in a 5 speed.

 

[Hammer]

With final drive ratio's on the 2 trannies so close,5sp-. 75 and the 6spd-. 73,I'd have to say to install a good quality overdrive unit for what you want. The only benefit to the 6spd is a extra lower hole for towing(if you ever get that low). By using a O/D unit it will allow for split shifts and a wider band of usage to apply it... ... Andy



BTW. . I forgot at add alot of the sled pulling guys prefer the 6spd because of the extra lower hole to overcome questionable track conditions.