Off Roading 3.07's off road

[MMiller]

Hey guys

Posted this question in the first gen area, and did not come to much of a concensus. So I thought I would put it here.



I purchased a 1990 dodge(see sig) and it has 3. 07's with a 3 spd 727. I will eventually run 38-40 inch swampers or boggers on the truck. My friends and I wheel in the midwest, trails, some mud, little sand, occasional rock not like moab though. I am pretty sure that the 3. 07's are gonna be tall, and the diff housings won't accept any other gears. I will probably trade for another set of axels.



I was wondering if you guys think that 3. 55's will be a good gear for those tires. Some buddies of mine run those size tires with 4. 10's and mild big blocks. BUT I think the cummins will make more than enough torque to run the 3. 55's. I always run low side on my 75 dodge and have had 36's on it with no problems(its 440 4sp with locker in front, and limited slip in rear).



This truck will still be road worthy, I can't justify having a truck that I can't drive to town. It ain't gonna be an interstate truck, or long hauls. But go to get parts, cruise, stop light abuse, etc.



What do you think?



Michael

 

[XJSuperman]

First, your diffs should easily accept a WIDE range of ratios. I am guessing you have a D-60 rear D-44 front?? If so you can put MANY different gears in there.

As for 3. 55's... . yes and no. Yes because you'd have a nice 'highway' crusing speed with 3. 55's and 38's.



NO... If you go with meats as big as 38's (or bigger) with 3. 55 gears I think you'd eat your transmission pretty quick. The cummins would handle it fine but the auto trans would get hot. If you never intend for it to be a 'highway' truck I would look at 4. 10's. You'd still be able to cruise at 60 MPH or a little better without the motor screaming... but you'd be REAL nice at slower speeds of in town driving... meaning the transmission won;t have to work as hard, plus you'd have a lower overall ratio for the offroad aspect. It's nice just to let the cummins idle over everything

This is mostly my opinion as the transmission will most likely be the weak point of the setup... 3. 55's will work fine most likley... but if it were me I;d go a little deeper.

 

[DHorstman]

Sounds like you have a Dana 61 front axle. I had a '90 with the same setup.



Local dana/spicer shop looked up R&P ratios and said I could get 3. 07,3. 54 and 3. 73 gears for that housing without changing carrier. Steeper gears are available but don't know what all is required, spacers, machine, work etc. Years back I ran into a guy who bought a set of 60/70s with 3. 07s. He had them regeared with detroits and 4. 88s.

 

[MMiller]

Yes the truck has 61 front/71 rear. According to the guys on the first gen forum, a guy can't get any other gears for these diffs. Sound like any more you can't even get carriers or R&P's anymore. I am pretty sure I can trade for a 60/70 for even, many guys said they want the 3. 07's for there trucks.



Talking to my friend that has 38's/410's with 460 his highway speed is already at/near my governed rpm. With a good tight torque convertor, I would think that the 727 would be the least of my problems. Before I do any engine mods the transmission will get beefed up.



I know if I used high side of the t-case I would hurt the trans, but I always wheel in low side. I am trying to take into account that a gasser will run from idle to 5,500 pretty easily, were as my cummins will only run to maybe 3000. I have to give it some gear to make up for the lack of rpms. That is why the 90 came with 3. 07', before dodge had an overdrive auto.



I was wodering if any body over here had run taller tires, other then on the pavement with 3. 55's.

 

[DHorstman]

IIRC, I also called DTS and they had the same answer as far as gears. They could get 3. 54s&3. 73s for the D61.



I'm thinking about making an open wheel buggy with a '96 CTD in it. I have a D60 with 3. 54s from a '92 dodge, 14 bolt rear with detroit. Not sure if I'd run a nv4500 or dig up a 727/np205 for it. Would run 40's on it.



Kinda funny how so many say the D61 is no good, but still want the same price as a 4. 10 geared D60.

 

[shorty2500]

i run 3. 54 right now with 38. 5s offroad, and i can't wait to put 4. 10s in. i recommend switching to 4. 10. i can still get around, but i noticed a big difference going from a small tire to a bigger tire.

 

[RCCola]

5.13s

I think with the thick carrier and a spacer you can run as deep as 513's Call Jim's OffRoad Center 865-482-6912 he has them on the shelf and has done sereral 61 regears



He is a TDR guy and can get you taken care of.



hmrdwn

 

[MMiller]

Shorty;



Thanks for the input, thats what I'm looking for. Wondering though, do you notice it more when getting the truck moving, or keeping it moving. I know there will be some difference because I am running an auto, I noticed your 5 spd. I will also be short on the top end, not having OD. What type of wheeling do you do? Any mods to enhance power? With higher hp do you still think 4. 10's to be necessary? I'm am just wondering. Haven't made any decisions yet.



Michael

 

[Philip]

Local dana/spicer shop looked up R&P ratios and said I could get 3. 07,3. 54 and 3. 73 gears for that housing without changing carrier.

Ask that shop if they will accept the gears back after you try to install them. I think you will find out that a 3:54 or deeper gear set will not work on the 3:07 carrier/housing.



I know for a fact you will not do this on the model 70 rear axle made for 3:07's.



Here is a link with some good reading in it.



https://www.turbodieselregister.com/forums/showthread.php?threadid=68679



That link covered the rear axle. I wouldn't bet one cent on the front axle being any different.



There are a few people in the first gen section with 3:54's and wanting 3:07's. Take the easy way out and trade. You might be able to find 4:10 gears also.

 

[shorty2500]

Originally posted by MMiller

... do you notice it more when getting the truck moving, or keeping it moving... What type of wheeling do you do? Any mods to enhance power? With higher hp do you still think 4. 10's to be necessary?...

i notice it when getting the truck rolling and when towing. just driving down the street/hwy, there isn't really a big problem.



on the trail, my 3. 54 gears with 38. 5-in tires causes me to get too much wheel speed at times. that's part of why i want to regear. there's no issue with respect to power or torque. there's plenty to go around.



the wheeling i do is dirt trails with rocks - big rocks and little rocks and everything in-between. i like steep trails with curves and trees that make me really try to squeeze through.



i have added nothing to my truck to increase power/torque.



again, these trucks can handle the load if you want to keep your gears the way they are. it just depends on how much wheel speed you like when you are trying to crawl up or over something. i have found that my minimum wheel speed (4-lo and 1st gear and idle) has been too high in some cases.



if your truck came with 3. 07, then 3. 54 might be good for you. it all depends on your transfer case and 1st gear ratios. mine is 2. 72 and 5. 61.



with 3. 54 gears in the differential, i multiply 2. 72 x 5. 61 x 3. 54 = 54. 02.



with 4. 10 gears, i get 62. 56. if you want, take your numbers and compare to mine. if you are lower than 55 with 3. 07s, i'd say you'll want to change your gears.

 

[MMiller]

Very good input. Exactly what I was looking for. I will be trading axels out. I am going to do this for two reasons. First enough guys want good 3. 07's I want somebody to have these for their truck. Second I want a real common setup, easier to get parts for far from home, even around home. No need paying extra $$$ for trying to get differnt gears in a set of axels that are peculuar.



I found a neat gear ratio/rpms spreadsheet and have done some figuring. I am still not decided on wether to go with 3. 55's or 4. 10's.



I just want to make sure I can still drive this truck with the 3spd, down the road at 55 without having to change to 3K gov springs. I'm sure I will do this, but I am not going to do it to drive 55.



Thanks again guys. By the way if has a 205 t-case. Its low is not quite 2to1, 1. 96to1 or something. Ain't got a big drop to low side compared to newer aluminuim t-cases.



Michael

 

[GO 4LO]

Couple things to keep in mind about the gear ratio / rpm spreadsheets out there: First is that they normally tell you to calculate off the diameter of the tire. If you have 40x17 Ground Hawgs, putting in 40" will not be correct. Measure the tires mounted on your truck from the ground to the centerline of the hub and then double this measurement. Second thing is that you need to account for not only the new gearing disadvantage of the tires but also the increased rotating mass and frictional losses. The formulas do not account for this - it can only be accounted by going to a slightly deeper gear or increasing engine performance. Think of it like spinning a 35" tall by 8" wide tire vs. spinning a 35" tall by 20" wide tire that weighs twice as much. You'll want a deeper gear or more some more engine torque for the second set, even though they are the same diameter.



39___ 3. 07___ 1454. 707692

39___ 3. 55___ 1682. 153846

39___ 3. 73___ 1767. 446154

39___ 4. 1____ 1942. 769231

39___ 4. 56___ 2160. 738462



Assuming actual ground to hub centerline of 19. 5", the above calculations should be correct for RPM at 55mph in 1:1 top gear with the given axle ratios. Hope that helps.

Chris

 

[GO 4LO]

RPM =



(336)*(tcase ratio)*(transmission ratio)*(axle ratio)*(MPH)

(2)*(distance from ground to hub centerline)



Here's the formula if you want it. Note that all the ratios in the numerator are for the gear you're in at the time, not the lowest available. So for highway travel, you'd use 1:1 tcase, not 1. 96:1 low, like a 205 has.

Chris

 

[fj40charles]

Don't know what your transfer case gear and 727 first gear is, but I would recommend you get at least 4. 56. I think 4. 88 would be a better choice for 40" tires though. Remember, the torque of the cummins is nice, but there are times where you need a lower gear because if you have to give it some fuel to turn the tires, good chance you'll make the tire slip..... You want a ratio that will allow you to crawl over rocks.



FWIW, I run a TH400/Atlas 3. 8 and 4. 88 r/p in my FJ40 behind a 350 motor... . It is fine for trail usage, but I could use a deeper gear for driving on the street.



Like others said, having a high gear r/p will just cause too much heat (from slipping the torque convertor) and will cause the transmission to fail quickly.

 

[MMiller]

Very good info guys, I struggle with the fact of needing the same gears gassers do. My buddy has a 460,C6,4. 10, and 38" swampers and a "stall" convertor of some sort. His truck on the trail stalls to 2,000 before it moves, why can't I use 800 rpm-2000 he is wasting? I am very impressed with the way his truck runs off road, very smoothly. Granted his big block runs to 5500, mine will only run to say 3000. I realize this is kinda uncharted territory, and that is why I am asking more people.





Michael

 

[fj40charles]

Originally posted by MMiller

Very good info guys, I struggle with the fact of needing the same gears gassers do. My buddy has a 460,C6,4. 10, and 38" swampers and a "stall" convertor of some sort. His truck on the trail stalls to 2,000 before it moves, why can't I use 800 rpm-2000 he is wasting? I am very impressed with the way his truck runs off road, very smoothly. Granted his big block runs to 5500, mine will only run to say 3000. I realize this is kinda uncharted territory, and that is why I am asking more people.





Michael

Stall of 2000 is too high. . By the time the transmission applies traction, it will spin the tires from too much rpm... . I would recommend a RV torque convertor which typically has a stall of 1100 RPM. I run an Allison tc behind a stock TBI 350 and it is fine... At a minimum, I'd get a "stock" stall convertor which is usually 1500 to 1800RPM.



I think as a general rule, you want around 100:1 (factor in 2x multiplication for torque convertor) crawl ratio. Some people have lower, but you run into problems with "driving through the brakes" for people without hydroboost.



I'm at 92:1 crawl ratio. . 2. 48 (TH400 first gear) x 3. 8 (Atlas II tc) x 4. 88 (ring/pinion) x 2 (torque convertor) = 91. 97



There is a fine line for what crawl ratio you need... Go too low like 150:1 or lower and it will work in very limited areas such as Moab or very technical high traction areas. Mostly on the west coast.



You'll want a higher crawl ratio if you wheel in areas where you have some water and mud... Tellico is a good example where you need tire spin to get over obstacles..... It is funny to see these west coast guys come out east and they try to crawl over everything.....



I think a good compromise is around 100:1... Remember with an auto, your actual crawl ratio can vary because the torque convertor will slip...

 

[therabbittree]

the crawl ratio with the auto is nuts! that x 2 at the end makes a big difference. . I have to figure it out for my stick and when I swap the trans out. . neat

Later

Deo

 

[MMiller]

Wow, brought this one from the dead... ... just an update, if you look at my sig, I went with 4. 10s and detroits in both axles. The truck has worked very well here in the midwest. There has only been one time I could not get the truck through an obsticle due to gearing. I had 3 of the tires wedged in and under downed trees and teh truck would not spin the tires and could not move forward. Any more gear and axles would probably started failing anyway. A reverese, and new line was picked and forward progress was successful.



I have a 47rh to put in the truck in the future,(its all purple) and when I do that I want to put a doubler transfer case kit in it, for the few times that I need more gear for technical parts of the trail. I'd still have a 2:1 for midwest wheeling, and then about a 4:1 for crawling. I'm hoping someday to make a trip to Moab and experiance this thing they call "slick rock". I pretty sure its a hole bunch different then the mud and moss covered rocks here in the midwest.



I'm happy with the 4. 10's and 38" swampers, its a good combo IMHO. I think 3. 55s would be a too tall. I'm not planning on any taller tires, but would like some 15-18" wide tires for better floatation. The 12. 5" wide tire dig plenty quick under a 6500# truck.



Michael