All,

I have a '90 F350 Crew Cab with a 460 FI w/4X4. I do mostly open road driving and hate the 10MPG. So, I bought a Cummins 4BT with a list of all the goodies to make 300HP and 525Ft/Lbs, more than my stock 460 and a hell of a lot better gas mileage (20+ easy). Plus, it is a lot simpler to work on, not computer controlled, and takes ALL ignition problems out of the picture.

However, I will still run into a slight problem. I was running nearly 3K RPMs on the interstate (70-75MPH) with my E4OD and 3:55 rear diff. I understand that this is mostly a towing rig, got it. However, when I do tow / haul stuff, you can't use OD anyway as it is too tall. A Half gear down would be perfect as third currently runs high RPM and OD won't hold.

Other than a $3K Gear Vendors overdrive, I am left with differential gears. I can put in a 3:08 which would be great for highway cruising and allow me to run high third for hauling / towing light loads. It would also be nice for towing, as the diesel would be in it's power range at the lower third gear. Now I end up putting my drivetrain in a bind for four wheel drive.

Dana made a D61 front axle for Dodge in a 3:07 that would be perfect, but I do not want to do a complete axle swap. How hard is it, and what is involved, in putting the 3:07 gears into the Dana 60 reverse D60 case? I know people do this and run 4:XX higher gears. What if I want to keep the same low ratio? Is this a 35 spline like my D60 monobeam? As a last resort, would a cut and weld work? Cut my D60 housing out and weld the D61 center into place? I have also seen where there is a 3:31 available for the rear 10.25 Sterling and have seen (Bronco.com) where some of the D61's came with a 3:31 up front. This would be better, but I cannot confirm the existence of this axle with this gear.

Are either of these D61s (3:33 / 3:08) reverse cut like my D60 monobeam?

Just asking some questions and trying to find out a cheaper alternative than a 3K Gear Vendors system that a lot of people on this site have said do not get them more than 1-2 MPG better.

Bob

 

why not go to 4:10 and use O/d?
the 3.07 dodge thing, i dont know a lot but i read it somewhere it was a 3.07 ratio meant to run in a 4.10 carrier. or something like that.. i'm not sure how they are interchangeable with the 60's
another thing with the tall gears is taking off loaded and backing up.. you'll be using low range a lot. loaded in soft stuff i mean, it'll be workin the converter and backing up you'll be going a too fast if you have to line up or get in somewhere tight.

 

The 4:11 would be good in towing or hauling, the 5% of the time I do that. It's the other 95% that I need the savings for. I drive 400 miles round trip EVERY weekend. So, for those times I do tow, I simply have the 3:08s and push the O/D button OFF and use the full range of Third. Anything above 2,200RPM in the diesel is wasted gas as that is out of its power and torque ranges anyway. That is why they are limited to 3K injector pump springs. Most 5.9 conversions start defueling at interstate speeds (70-75mph) whereas a lower rear gear will drop the fuel requirement without sacrificing power or torque and stay well within the power band.

Bob

 

This doesn't sound right.

My bronco runs at about 2300-2400 at those speeds with a 3.55 rear end. Your tires are taller than mine, so that lowers your revs.

Are you sure you don't have 4.10 gears in the truck?

 

I do not have really tall tires. They are General brand and slightly taller, no more. This is a highway truck. I run from Ft Campbell to Somerset, KY every weekend and I run about 2,700-3000 on I-65 and the future I-66 all the time. I definitely have 3:55 in the rear. Diff code is 3:55L, for limited slip. Either way, at 70-75 MPH with the diesel I will be above the power and torque area. I am trying to get the RPMs down to 1800-2000. The 3:31s would do the trick but I am not sure of a 3:31 front D61 carrier and diff setup.

 

I'm still not understanding why you are not trying to bring your truck into optimal RPM in OD? 300 horse and 525 ft lbs should be plenty of power to tow light loads in OD.. i do heavy loads in OD with a turboed 7.3 IDI.
i have 4:10 and i run around 2200-2400 in od at that speed

 

Because it is not good on the life of the transmission. The tranny heats up a lot while towing in O/D due to slippage (low engine RPM = low trans pump pressure + high torque = slippage). I would rather tow in Third (med eng RPM + med torque = med pump pressure = little to no slip) and drive empty in O/D (low rpm, low pump pressure, low torque and no slip + no trans heat).

 

E4OD is meant to pull in OD. if its building heat and slipping then it needs a rebuild. a good shop will reroute some of your oiling and increase pressure in 3rd and OD therefore eliminating such problem from occurring again. still my point is valid. run your truck in OD with 3:55 empty should result in 1800-2000 area for RPM and if you feel better pulling in drive then so be it. but it sounds like its slipping even empty. 3000 in drive seems a bit high.. especially with a lockup convertor

 

A new tranny is going to go in regardless. It will have to be beefed up as this is and early E4OD, not the stronger 4R100. Plus, it will have to have the diesel torque converter with the low stall speed, not the higher gas one. I have been looking at Level Ten, Monster in a Box, Performance Transmissions and others. You are probably right as this is the original tranny from '90 and it has held up well. Your vehicle may be turning 1800-2000 in O/D but I am positive mine is not. More like 2,200-2500 if that low.

Still, the original question of the 3:31 and 3:07 for the D61 remains unanswered. I am just looking for options other then the Gear vendors 27% overdrive for $2,995.00. It is easier (and cheaper) to experiment with rings/pinions than and overdrive unit that you cannot send back.

End result would be to have lowest rpm in O/D for gas mileage while running empty in the diesel power area (1700-2000) and still have power in full range of Third for towing while getting slightly less mpg but still better than my 460 gasser. If I have to sacrifice something, I will sacrifice towing in third for mpg as most of my driving is open road, not towing so the little time I do tow, it will not affect my wallet as much.

 

ok. D61's share the same carrier as SOME D60's they run a different offset than the 60's. i dont know if they were reverse rotation. i could not find new 3.07 gearsets although i know the dodge diesel's had them. i could however find a 3.31 gearset. I found this on Randy's Ring and Pinion. This seems so redundant as the problem is clearly with the trans which you recognize. If you fix the trans you will get exactly what you're looking for, 1800 or so RPM in OD travelling down the highway. It will also get you runnin in Drive, (3rd) in the 2500 rpm range for your towing which you do 5% of the time. if you by some miracle get this 3.07 ratio to work then you'll be runnin close to where you want to with a *****ED trans, then the trans blows and you get a rebuilt, which will now make you run too LOW of rpm in OD and likely too high of rpm in Drive (3rd)
why go to all that trouble to rig up something to make your truck run where you want with crapped components.. with a crapped trans you are throwing fuel economy away just because of the fact that its crapped.. isnt that why you did all this?.

 

What I am saying is that once the tranny is replaced, I have no certain idea what the final rpm will be due to the fact it runs great right now with some slippage (after long drives 200+ mi and then enter city driving). It does great around town all day long. I will be substituting a new transmission, a diesel torque converter, and a totally different engine (and tires play a factor). There are absolutely no guarantees that it will be as you say or as I say. Mr. Murphy always has the final say and being as there are 4 variables in this equation, all bets are off. I just want to have options BEFORE I do all of this so that if it ends up not as expected, I am not behind the power curve trying to map out my options. It would be absolutely great if, after all is said and done, it is as you say.

I do appreciate the input though and I am grateful. I just like to have Plan A, Plan B, and so forth Just In Case...... If I ever have a project go perfect as planned without several trips to the hardware store and auto parts store, I get worried because something bad is going to happen. If anyone else has been able to do this, please let me know how it feels.

 

The code might say 3.55, but the RPMs don't. I say you either have serious transmission problems (e.g. not shifting into OD at all) or much shorter gears than you think you do. 3000 RPMs at 75 MPH doesn't sound much like an overdrive transmission with 3.55 gears.

People change gear ratios in trucks pretty often. This makes the tags and stickers wrong. I've also seen them wrong from the factory. If it were me, I wouldn't take it for granted. I'd be out there jacking the truck up and turning the tires and counting revs of the driveshaft.

Something just doesn't add up with your truck.

Going from 3.55 to 3.31 is so small of a jump it's not worth it. I also would double-check your math if you think that particular gear change is going to net you 1000-1200 RPMs.

The E4OD has a locking torque converter. There shouldn't be any slippage at all while in OD. If you have a tranny temp gauge and you show it getting hot on the highway, then you have tranny problems.

 

I don't think 3:31s will net a huge change but anything that will help lower rpms will help save gas and money in the long run. Remember, I am trying to see cheaper alternatives other than the 3k overdrive. Just keeping options open.

As far as locking up, it does lock in O/D. However, after a long drive (200 miles at 70+ mph), when I enter the city limits and hit lights, the tranny shifts "funny as if it is slipping into gear rather than shifting (soft shift, not firm). As I said, it is the original tranny as far as I know with over 140,000 miles. It just has to last a little while longer unti the engine is ready then I can replace them both at the same time. I do not want to put a new tranny in with a diesel t/c behind my gasser while I am building my diesel. Plus, the bolt pattern is different.

I plan on putting in a temp gauge when i redo the electrical for the engine and stand alone tranny controller but as for now, it does not have one.

If someone changed the axle r&p, they may have. I am only going off of the tag that seems to be original. I am deployed right now or i would do all these suggestions. I just want some other options in case it is indeed a 3:55 and the rpms are high.

I know I drove different speeds and wrote the corresponding RPMs down as I increased speed and they were high.

ANDYM....what rpm are you running at say 75 with your 3:55, E4OD and 351?

 

About 2400 or so... but remember my tires are roughly 3" shorter than yours.

I think you have a couple things going on. One, your tranny is on its last legs. A fluid and filter change might help, but if it's been neglected and the fluid is in bad shape, that will likely just cause it to fail sooner. I also really think you have 4.10 gears in the truck.

 

Could be you are right. I just want alternatives in case it is 3:55s. I know the tranny has to be replaced. To keep it in there with that many miles, the fact that it is an original E4OD and not a strengthened 4R100, I would simply tear it up with the diesel torque.

If it is 4:11s, easy problem to fix. I will, of course, wait to do any gear change until after the tranny is replaced, engine installed and numbers crunched to see what the final rpm is as to whether or not I need to change anything. Just doing a "what if".

I would still like to know about the D61 as a backup.

I am supposed to talk to my wife any time now and I will have her get the tire size and height for sure.