Dave, would a gearvendor underdrive or overdrive unit be a solution too? That might give at least a little more selection for the speeds you need to run.

 

Dave, you make some good points.

I was confining my comments to engine RPM, not vehicle speed. There' no way to stay "in the box" on RPM at high speeds without seriously increasing the load factor.

Cruise RPM even higher than 2200 might be desirable in the cases where you want to tow at faster speeds (to me, over 65mph is FAST when towing, never mind the ranchers doing 80mph with livestock in the back).

Even thought you will have less torque available at RPMs over 2200, as the engine is lugged down from higher RPM, you will see a torque rise to peak torque as RPM heads down to 2200. So maybe a cruise RPM of 2400 or 2500 would be ideal in the most extreme cases. This may also buy some kickdown margin in the trans, making it more likely to try and hold the gear. Afterall, the kickdown is more likely if RPM is lower (and the next lower gear is within the power range). If you're already running 2400rpm, a kickdown doesn't really buy you anything but lower tranny loads.

If I towed with any regularity at all, I would embrace the steep gears and be done with it.

4.30 would be ideal, but 4.56 isn't that much lower. Remember that as the numbers get bigger, the each .2 or so of ratio means a lot less. In otherwords, going from 3.30 to 3.56 is a MUCH bigger change than going from 4.30 to 4.56.

JH

 

I've looked that way years ago when US Gear had what I thought was a good unit. Gear Vendors is the survivor it appears.

I would rather gear it for towing since I'll do very little driving without the trailer in the future. I getting 19 on the highway with the wing off and about 16 with it on.

Thanks for the info.

Dave

 

Well I towed from Virginia to Pennsylvania last night and temps if towing in 5th around 2000 rpms the EGTS will remain at about 1000 or a tick under but if I kick it into 6th gear and tow at 1700 rpm the egts will shoot up to 1200-1300 while pulling a hill so its definately better for me in 5th gear around 2000 rpms and speed of 59 mph or so! Not sure if thats optimum for fuel mileage though?

 

Running up I-81 from Roanoke to Gettysburg last year the most I saw in EGT's
was 1100 or so at 63-65 mph in OD with the Edge set on Tow. (730 ftlbs Tor)

My mileage did not change going up I-81. Same as on 95. It will be interesting to see what your mileage is. I really don't think it will be worse than running in Sixth.

Dave

 

Lots of good info and good opinions so far..... Optimal towing RPM's as mentioned are going to be based on, weight of the trailer, desired speed, desired fuel mileage, towing terrain/conditions, and your EGT's. Some guys like to tow at dangerously high speeds and some like to tow no more than 55, so how can you use absolutes when free will is a variable. I tow heavy in 4th gear on flat ground at about 2100-2300 RPM at 55-60 MPH with EGT's at 900*-1000* but if the engine load is low enough for the EGT's to drop below 900* then I may use 5th gear but I watch my pyrometer closely to determine engine load and if I need to drop back into 4th gear. Thats what works for me. There's no point in stressing the engine and driveline just to keep my RPM's lower.

Running higher gear ratios will reduce load on the engine by increasing leverage but determining if you need the higher ratio will have to be based on how much you tow, how often you tow, and if you dont mind the higher RPM's. Also, I would stay away from auxiliary gear splitters. I'm not sure if they've increased their strength but I've heard that they aren't that reliable behind the Cummins. Plus they cost A LOT of money, not including necessary driveline modifications. Your fuel mileage would have to increase substantially to offset the cost.

 

Not optimal for mileage. If your truck is stock, then go ahead and let it pull the taller gears-- you won't hurt the engine any, and it will give better mpg.

If you're not stock, then you might overload the driveline a bit and downshifting, while a negative for mpg, is overall a good idea.

jh

 

Good post from Katoom. There really are no absolutes for this type of discussion. At best, we can only speak in generalities and relative comparisons-- A might be better than B under this particular condition, but not under these other conditions, etc etc.

I would also stay away from an aux trans. I don't know anyone that's gained enough MPG to get one to pay for itself yet.

One advantage of the new 'Cummins 800' calibration is that the 800 lb-ft really reduces downshifting and should help towing MPG.

 

MY neighbor has a GV unit in his 99 5 spd with 4:10s he got a consistent 14 mpg empty now with th GV and a set of rv275 he is getting 19 empty but towing it is still 10-11 mpg but he can find a gear thats just right and works the drive train easier. personally I don't like it other then the MPG increase
+ it was $6000 to install and 3 years later it was $6000 to install a new one because it failed .I really like my 6spd with 3:54 and 285-75/16 it gives me just a pinch more speed & lower RPMs when i run in
5th and I usually have 17,000# behind me

 

Are you saying he spent $12,000????? Thats just..........well you can fill in the blank.

 

That sounds a bit pricey. I had a GV on an 03 3500 auto with 4:10's. If I recall correctly it was about $3k installed. It worked flawlessly pulling with the 5'r at the time. Never had an issue. In regards to improved mpg, maybe some, but not enough to ever "break even". Only draw back, can't use it in 4X4, the US Gear you could as it went between the engine and tranny.

 

Excellent thread! Bookmarked.

 

GV will exchange a wasted one for rebuilt for $750.00 (in 2006) plus a few hunderd for a tranny shop to swap it.

GV is really good on the 5 speed as it makes a perfect 7 speed.

GV is not very robust. Pretty lightweight and somewhat quirky as to when and how it operates. None the less it is an option. Mine went 150k miles.

Even though you may tow in direct and not OD on the GV mine failed the mainshaft so had to be towed.

 

I tow very heavy most of the time and the gear vendors doesnt really help in the towing department. I tow the dozer and trailer and they weigh in at 24,000 lbs. I also tow this in AK and see alot of mountains. The gear vendor helps in the 3rd to 3rd over instead of hitting right into fourth. Instead of a 1000 rpm drop, now I have a 500 rpm drop and I can pull the hill and actually accelerate. I will tow the gooseneck empty with the GV on because i can tow it at 70mph and turn the GV off and the truck will slow down. That is 8k for trailer and 8k for truck.

I do have a six speed manual so my .79 overdrive is awesome for towing heavy. 60 mph is 1900 and I sometimes wish I had the 4.56's for towing knowing that I have the gear vendors, but I will just upgrade for the 4500 next.

My mileage went from 13mpg at 65mph up to 18+ with it on. I run 1700 rpms at 70 and I have done the math, it is real close to what the six speed auto runs at 70 in sixth gear.

I have broke mine at 40,000 miles with the trailer hooked up and I wish they made a stouter unit. It cost me 2,700 to install and buy the unit, and the replacement unit was 850 bucks. I did it all myself. Pretty easy to do.

The main reason was mileage, but the truck now just hums or cruises instead of winding out down the highway. "When" this one breaks, I will replace it either with a new truck, or replace the GV unit again. Truck is paid off so...

 

I was towing this weekend, and noticed that I could keep the EGT's to 900 degrees/ and eng temp just over the 190 mark, 2100RPM's at 55 MPH in fifth. When I would speed up to 62 MPH in sixth my EGt's went to 1000 degrees, 1800 RpM's, and the coolant temp would be just under the 190 mark. Just wonder which is better for the drive train. What do you all think?