Transmission Gearing vs Rear Differential Gearing Question

[ChevellRCR]

Ok so I'm rebuilding my truck. 94 crew cab dually turbo. Going back to an E4OD from the zf and looking at gearing. This truck pulls fairly heavy loads at times. Currently I have a 3.55 gear in the rear but considering going to a 4.56 now that I have an option for a double overdrive. I was using the grimjeeper website and came across something interesting. I can run my 3.55 with an under drive unit and get the same RPMs basically as running the 4.56 gear with double overdrive. Question is which is better on the truck and transmission? Or will it not care where the deep gearing is coming from? This is with USgear Dual range units as I can leave them in the desired range from the start. Please see the pics below.

4.56 with straight gearing on left and with double overdrive on the right

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3.55 with straight gearing on the left and with underdrive on the right

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[IDIBOBS]

What sized tires? Personally I’d gear the rear end to the tires and skip the over drive unit. You can’t tow with it and a tuned empty 7.3 can already get into 20lus mpg. I don’t thing the over drive on top of the E4od over drive is gonna help much. Are you towing a lot of the time, how much weight do you tow or haul?

 

[Clb]

Towing final drive all day long =lower the better...

Slushboxs usually have higher final drives due to torque converter losses (yes I know but slippage IS loss).
Fwiw
You can have to much overdrive, sorta not applicable to underdrive ..
I am still looking for a ride in an idi getting +20 mpg tho.
Carry on

 

[Black dawg]

Is your us gear od or under drive?

 

[IDIBOBS]

My transmission locksin 2,3 and 4 but I’d never tow or haul anything on OD not enough line pressure. Just hear the rear end right for your tire size and go to it. And yes I run a 38g rear and a 19g front fuel tank for a total of 57g. I get over 1300 miles per fill up when I’m empty. It’s easy if you’re tuned right and keep your foot out of it and slow down. Now with the the 8500lb horse trailer it goes down to about 850 miles per fill. I hear trucks were built to go at most 55mph.

 

[IDIBRONCO]

I hear trucks were built to go at most 55mph.

They were. The national speed limit was 55 MPH when our trucks were built.

 

[Black dawg]

55 for any distance would be painful.....drove 10hrs the other day at 65, would have taken a couple more at 55....

 

[DaveBen]

I remember driving at 55 mph during that time. I would eventually speed up to 65+ to get back to my standard travel times. Almost every one was going 65mph...

 

[ChevellRCR]

To answer questions. My tire size is stock and the rpm to speed ratios above are with that stock tire size. 215/85-16

I have a few US gear units. Under and overdrive.

The goal is to keep the truck in a good rpm range say 2600 rpm at 73 where I tow a lot on the interstate. That way my turbo is always on go. This truck will be towing boats, equipment trailer, or my camper at times when I'm not using my International 4700 (DT466 and Allison 3060 on air ride suspension). Loads are between 4000-20000 lbs. When I'm not pulling the loads I figure the double overdrive or the 3.55 can be useful as it lets me slow the engine down at speed when not under load.

Fuel is not at issue as I make my own and I dont care about economy. Truck has factory size tanks underneath but a 95 gallon toolbox tank and when bumping pulling say my boat to the keys I can add another 65 gallon tank to the back of the bed. For reference a round trip to the keys for me is about 1700 miles.

I am leaning towards the 4.56 gearing but was just curious what you guys though would be the better way. I'm not worried about pulling in overdrive. I have massive cooling and gauges. I have a couple of these transmissions and they are build heavy with aftermarket valve bodies and billet converters. So i have a spare and if it comes to it I'll get Brians truck shop to upgrade it further.

 

[Black dawg]

4.56 with e4od, and the us gear od would work out pretty good. I will say though, the usg units I have been around tend to run hot with high driveshaft rpm and extended time. The one I have now, with 355 gears , and stock height tires will start smelling like gear oil after a couple hours at 75 80mph.

 

[ChevellRCR]

4.56 with e4od, and the us gear od would work out pretty good. I will say though, the usg units I have been around tend to run hot with high driveshaft rpm and extended time. The one I have now, with 355 gears , and stock height tires will start smelling like gear oil after a couple hours at 75 80mph.

Thank you for that info. I haven't heard that one before. Interesting.

I have a 7.3 excursion I was going to install one of the units on eventually. It has the 3.73 gear and I figured slow RPM on it could be nice for an interstate cruiser. Going to have to watch those temps I guess. Thank you

 

[Black dawg]

I have wondered if there was something wrong with it, but 10 years now, and the oil still looks new.

 

[ChevellRCR]

I'll def keep an eye on it. Prob run synthetic as well.

 

[Cant Write]

A real geek of an engineer once told me to get all your gearing done as close to the engine as possible for efficiencies sake.

Besides Ford’s 4.30 gearing option in their super duty gas rigs, it sure seems everyone is going for lots of trans gears and lower numerical diff gearing.

So it must be better to go 3.55’s with the under drive?

Looking at your pics, I like the E4OD and 3.55’s playing between 3rd and 4th depending on terrain. But take off gearing may suffer. So I would choose the 4.56’s and play between 3rd and 4th with the AUX OD engaged. I assume you can tow with it engaged.

Thats my 17hr’s of work tired eyes looking at it. I have no stake in the game and not much experience that heavy.

 

[ChevellRCR]

Thank you Cant Write. I know more gears are better. Hence why semi trucks have so many to get the load moving. You always have a gear for the speed you're in to maximize pulling power. I've never heard that about getting the gearing as close to the engine as possible. Would love reading more about that.

The overdrive and underdrive units can be ran in all transmission gears. After engaged they have no min speed and they have more weight rating than I will ever pull.

Speaking of take off gearing though. I thought the same thing but if the underdrive is engaged with the 3.55 gear as I would leave it when towing... The final drive ratio for first gear is 12.02. The final ratio with 4.56 in first is 12.35. Not a huge difference.

Get some rest.