Off Roading 3.54's and 38's - need some thoughts

["Mad Max"]

... okay, okay... before y'all think I'm looney, read the whole thing



The Truck: '78 Ramcharger (ridiculously modified) - 6BT Cummins, 518, 203/205 doubler, D60/D70, 3. 54's.



Currently I've got 35's, and it was built that way with a big block, 727, and a 205. Worked well enough, but I decided to upgrade to the 6BT, 518, and doubler. So now I've got double the torque and double the gear reduction. With the Cummins I've also now got a heavier truck, to the tune of about 7000 lbs total. And now... the 35x12. 5's are, well,... . small.



I'm getting bigger tires either way, and with the 6BT I know the truck won't hate the 38's with the 3:54's. My question is... . and please 'dig' my meaning... . will I need to upgrade to 4:10's sooner... . later... ... . or at all????



I'd love to do 4. 10's now, but can anyone out there in off-road land give me some 'reasurrance' that I'll at least, for now, enjoy the 38's for at least one season?



Engine power output will be +/- 300 hp and 600 tq in the end. transmission is fresh with hard core 6BT rebuild (converter, v-body, kit).



Also, my '93 Ram has 3. 54's and 35's and it loves 'em. 38's are a little taller but not a huge, so I know the hole shots may feel a bit slower but I'm guessing with a nice fresh transmission it'll feel purty sweet on the highway, and the highway is one of this rigs' primary play grounds. Add the doubler (4-1 ratio) and I feel it'll crawl pretty good too. It's being built to cruise as easily as it crawls.



See I'm not so crazy after all... ... ... right?



- Sam

 

[FordCummins]

I ran a 94 F350 with a turbo charged IDI 7. 3 (not the PSD) It was mildly turned up, with 3. 55 gears and 41" tires (only for a little while) but long enough to tow with a GCWR of 20k lbs from IL to NY. It did ok, obviously taking off was rough, and some of the hills seemed a bit long, but it did it just fine.

I would think you would be just fine. For getting into offroading, (depending on what kind you do) a switch to 4. 10s would be nice, but for now you should be just fine.

 

[DonCurley]

Normally, I would recommend at least 4. 56 gears for 38's, but that was from the days of my dealing with small gasser 4x4's. When I got my '07 3500 last July, it came with 3. 73 gears and I immediately transferred my 35" Toyo Open Country M/T's over from my '07 Power Wagon. I thought I would likely need to put 4. 10's in the 3500 then, but I had so much low end torque and HP out of the 6. 7 CTD, that even when in 4W-High with a camper on board, off-road performance was no problem. Based on this, I would at least try the 38's first on your rig and then see whether you need to do any regearing after trying it out for a while in a variety of on-highway & off-road conditions.

 

[MickeyMiller]

Don,

I agree with your observations... I do a fair amont of off road and I have yet to be power stalled. I even rarely need low range/1st gear... spend most of my time in Hi-range. The only reason you need gears is if you don't have enough engine!

 

[MChildress]

My son put 38's on his 97 F350 Powerstroke with 3. 55. I told him not to pull with it for fear of lugging the motor until he got lower gears (4. 10 or 4. 55). He ran it for almost a year with an auto staying out of OD until on the frwy. He got a tree clean up job and sold the fire wood and while delivering the trailer loads of fire wood burned a hole in number 1 piston. We pulled the motor and found hair line cracks in all the pistons. That was a rebuild 100,000 miles early. We have 33's back on her now. Just food for thought becareful.

 

[Moab4x4]

The only reason that I could possibly see needing lower gears with that doubler is for LOW LOW speed crawling. I would not swap gears yet.



The calculations I get show the 38's running you only 8. 6% faster than the 35's.



Running an online gear calculator with my truck with the Getrag 5. 53:1 low, it looks like I am getting ~2. 53mph/1000rpm with 33's. That is fine for almost everything. HG makes me a little nervous with the stick at that speed just cause I really don't make power until I get to 1800rpm.



IIRC the 518 has the 2. 45:1 low.

Plugging that into the same calculator, with the doubler, I have you at 3. 03mph/1000rpm with 35's and 3. 29mph/1000rpm with 38's. This is with a nonexistant lockup converter. Given that converter torque multiplication, you should be fine for almost anything you are going to want to do with Nacho.



On the top end, I am at 65mph in 5th at the magic 1800 best mileage rpm, 69 with 35's.

I have you with the . 69:1 OD at 77 with 35's and 83 with 38's with full TQ lockup. If you reduce those by ~10% (pie in the sky guess) you are right at 75 with the 38's and 1800rpm. Right where I would like to be.



Keep the 3. 54's, I think you will really like it there.



Ken



(this opinion is worth exactly what it costs. )

 

[dpuckett]

I agree- keep the 3. 54s. With your doubler, you should have no issues with torque. The only real reason I could see for even needing the doubler is in real tight corners where super slow speed would be needed, but not necessarily power at slow speed. The only time I could think of needing a lower range would be backing a 8ft trailer into a 8'3" hole.





MChildress- did your son have a pyrometer on his PSD? The early ones liked to run kinda hot from what I hear. The tires probably didnt help, but I doubt they killed the engine, either.



Daniel

 

[AVerHage]

38s and gears

I'm running 38. 5x16s on my 93 cummins with Dana 60/70 and 3. 54s, I thought they'd be to much but with all that torque its about perfect, only mods I have is "turning it up" and 4" exhaust with a Detroit locker, all I plan on changing is the turbo housing and injectors. Now I do have a 5spd so your torqueflite may be a little sluggish, but personally I'd have the trans. built and power up the engine some more, remember lower gears= lower MPGs

 

[BPitts]

if it's a trail rig get at least 4. 56's or 4. 88's

 

[DonCurley]

trail rig or not ... if he doesn't need lower gears, then why go to the trouble and cost of regearing?

 

[Moab4x4]

if it's a trail rig get at least 4. 56's or 4. 88's

If it weren't for the doubler I would agree, but doubling that 3. 54 down to ~7:1 should be plenty slow.



I'm running 3. 54's and 35's in my 92 CTD and it is a little tall, but I've had no lack of power up some pretty gnarly climbs.

 

[AVerHage]

Let the gas pumpers play with low gears we don't need to

 

[bjonesAZ]

What tire pressure will you be running on the road?



Bob

 

["Mad Max"]

Little bit of an update - I've decided to split the difference. I'm going with a set of 375/65R16's (36 x 15. 5), Mickey Thompson MTZ Baja Radials.

I'll have them on the RC in about a week and I'll post back for sure with how it feels. They should feel, and look killer

- Sam

 

[Forrest Nearing]

on a stock-ish truck it's not really any big deal, but on my '98 when I went from 35's to 33's, the truck felt a lot better.



I've got 35's on my '04 with 4. 10's, and it's a little much, but not bad at all. I am always in the powerband or very close to it. I will probably put 37's on it when it comes to buy new tires though