I thought I wanted one of these for my '05, but then I searched the threads here and now I'm not so sure. Where I could really use it is pulling my 14K fifth wheel and having a split between 6th and 5th when downshifting. I've hit many a grade where I am in 6th, run down to about 1,600 to 1,800 rpm, and go into 5th, and then I'm over-revving and have to slow down. I do not want to lug the engine in 6th trying to maintain speed.

I could also use it running empty, but I really don't run this truck empty that much (maybe 25% of the time). It's usually hitched to something. It would be nice though to be able to run 75 and 80 empty without hitting 2,200 rpm.

Anybody had experience with these units? Worth the $ or not? It will take me a LONG time to recoup the cost in fuel mileage. I'm sure of that.

 

What do you consider as over revving?

MikeyB

 

No expierence with one of them. I looked at the US Gear version a couple years ago. They don't make it anymore. My 2000 dodge had a 3:55. I was going to purchase the underdrive version so I would have a 4:30 ratio for towing by shifting into the low range.

I did not want to split the gears when shifting, just put it in low range and leave it there. Shifting those things, kills them.

I just looked again for the 06 auto I have now and its still not available from US Gear. Have not looked at Gear Vendors yet. The 48RE is weak on durability. My personal opinion is running a 4:30 will get me 65mph tow speed
at 2200rpm (in OD) which is were you want to be. That will keep the tranny temp down and keep load managable with the tranny.

Durability is not a problem with the NV5600 having drove one for 203,000 miles. So if you have 3:55's (Don't know what you have, no signature) I would consider slowing down when towing heavy and run in 5th gear which will give you 22-2300 rpm's at 60mph. 55mph gave me 13.5mpg at 55mph when towing the fiver (RPM's are up better mechanical advantage, It like having 4:10's so to speak). I got 10mpg at 65 in 6th. Lug Lug Lug. They say it don't but I think so. Ive heard as long as your in the torque range its not being lugged. Cummins says 2200-2300 rpms for hwy use using a target speed of 65 mph.

Most RV tires (ST) do not recommend towing faster than 65.

In Florida 65mph is the speed limit for a towed RV.

http://www.powerspec.cummins.com/sit...ng/gearing.htm

Select ISB for Engine type.

EDIT:

I had to come back and add this after running some calcs. This is going to sound funny, but, if I or you changed our rear ratio to 3.42 both of us could run in 3rd gear (48RE) at 64 mph (5th gear NV5600) at 2300 rpm's. I think for towing that would be great.

In the case of a NV5600, 6th gear would be 1800 at 68mph instead of 70mph at 2000 (3:55's) There is some real savings in fuel economy when your running under 2000 rpm's in OD.

Dave

 

Anytime I'm getting up over 2,300 or 2,400 rpm, I don't like it. But my main problem is the speed I lose going from 6th to 5th. It seems I always dump down to 50 mph when I downshift, then I'm trying to regain speed.

 

I wouldn't worry about that. Keep it in 5th and let the motor do it's thing. I'll drop out of 6th when the speeds starts to fall below 65mph when hitting a grade. Help keeps the EGTs in check. Also takes the strain off of 6th gear (the weakest gear).

MikeyB

 

I agree with MikeyB but I fixed the same thing with my 315 tires. It knocked 10% off my tach and reduced the rev in 5th when you downshift. Runs right at 2000 at 70mph. Some suggest you lose your low end torque but in 1st it really isn't noticeable to me.

 

I'd think about that, but I have a dually. My tires are actually getting close to the point that I need to think about replacing them soon. I'll probably just stay with stock size though, and maybe a little more aggressive tread.

I guess I'll dump the over/under idea. It sounded too good to be true. Back to the drawing board.

 

I don't think you can get a 315 on your dually but a taller tire would work, maybe a 75 series. Someone on here should know. Just look for 10% taller than your stock ones. I checked today and I'm at 2400/65mph in 5th. Your G56 may be a bit different than my NV5600.

 

I have the NV5600. Just missed the cutoff when they switched to G56, and glad I did.

 

Good deal. I like mine too, and it shifts smooth as silk.