So this is my first diesel. I've had it about a year and am getting to know it pretty good, but one thing I've always wondered about is my second overdrive's effectiveness with my high gearing. I've got a Unitrax/Gear Vendor/Laycox divorced overdrive with an auto and 3.55 rear end gears. It all came with the truck when I bought it. I know that when I'm driving in double overdrive it seems to run fine and smooth, but it also runs at pretty low rpm. I think around 1500 at 70. I'm generally on relatively flat freeways, so it doesn't seem to stress the drive train, but I wanted to get any input you guys have on this. Is it hard on the truck? A good thing? It's really nice to have the flexibility of the GV overdrive when towing with the transmission overdrive off.

From what I've heard those overdrive units do stress the main transmission. Kind of like oversized tires on 3.55 gears it still adds stress to the driveline but in your case to the input shaft.

No doubt it does stress the drive train some but my biggest concern would be lugging the engine.  I wouldn't use it towing anything heavy...  listen to your engine.  If it is lugging, you need to shift down.  Such low RPM is out of your power band. 

Gear vendors units are very strong for pulling. They aren't to be used as an engine brake due to their conical clutch.

They are self contained and take about 1 hp to run the pump. Use non synthetic lube btw.

As for straining the trans? Yes, it strains it just like a higher gear would... HOWEVER! the unit allows you to split gears and essentially have a 8, 10, or 12 speed that is quite reliable and will put less strain on the drive train when driven right.

The units also have a speedo cable/sensor built in and a good controller unit.

I wish I had one in my truck. I will probably put one in my barracuda at some point.

Edited June 14, 20159 yr by CSM

I have installed several of these when working at the gear shop, they are virtually indestructible units. Did a couple behind autos but most went behind manual trannies, I know of several guys who even power pull with them with great sucess. Just as a daily driver cruising down the road with 3.55 would be fine but one would need to do as CSM stated and find the ratios and play with different gear combos even with an auto, don't be afraid to manually shift that auto to find the sweet spots.

Optimal gearing would place you around 1,800 to 2,000 at highway speed for best results. Another way is the best gear ratio will produce lower EGT's. But the wrong ratio either too low or too high will increase EGT's.

So given the choice, unloaded on the flat freeway of either

1975 at 70 with trans overdrive only or

1500 at 70 with double overdrive

The higher rpm is the better choice?

I would pick the one that yields the best mileage, neither is going to overload anything you have without a load on it. Might even try GV OD engaged with main trans od off even.

GV on OD off bumps me to almost 2200

That's what I've used when towing my 7k trailer on flats.

The only thing that I would worry about is with that low RPM you are hammering the drive train pretty good. One of my friends has the same setup on a 12v and has knocked out 5th gear 4 times and from my research the low RPM was the cause. Some where I remember reading from Dodge that you should not pull at 1500 RPM or less and even on flat land at 70 MPH you are pulling it fairly hard, compare your EGTs and boost on both double over and over and you will see the difference. With your setup I would only use it to split gears but not 5th

Pulling and just driving unloaded are totally different venues even with an extra OD runit at 1500 and see what you do for mileage, don't worry about it, if you didn't want to put a load on it sell it and get a 4 cylinder  . 5th gear was an issue and it was not from pulling at low rpms it was a piss poor lock nut design from New venture, the engine harmonics didn't help from the Cummins as GM ran this same tranny for many years as well with no issues.

The only thing that I would worry about is with that low RPM you are hammering the drive train pretty good. One of my friends has the same setup on a 12v and has knocked out 5th gear 4 times and from my research the low RPM was the cause. Some where I remember reading from Dodge that you should not pull at 1500 RPM or less and even on flat land at 70 MPH you are pulling it fairly hard, compare your EGTs and boost on both double over and over and you will see the difference. With your setup I would only use it to split gears but not 5th

I would never pull in double OD, and fully recognize that 1500 rpm is too low to tow with (my exhaust temps would start creeping up pretty quick too).  1500 is only a question for me unloaded.

Listen to wild and Free, i think your gett'n all worked up over nothing, the GV doesn't hurt anything. just use it the way its most efficient, either GV on and just manually shift your main trans overdrive push button or down shift on inclines, 1500rpms should be ok on flat country if engine is not under much load with your set up your still in bottom of power curve' ;'watch your gauges' let the truck tell you what it needs, if your cruising at 1500 and you feel engine pull down ,egts go up meaning your incline on the road your on is increasing, "downshift, if you see your frequently down shifting just leave it out of main trans out OF  OD overdrive till levels out, same with transmission temps"watch it and downshift if needed to reduce heat

I put the same set up in my 1st generation 3500 with 4.10 gears. Fuel mileage did not change at all in any gear. I carried extreme loads over a 6% 3 mile grade and the gear splitting was the only benefit. Waste of money and more junk in my drive train.