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Towing and rear diff ratios

Discussion in 'Differential & Driveline' started by SirRobyn0, Jun 25, 2020.

  1. Blue Ox

    Blue Ox Turning Diesel Fuel Into Fun

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    No, I never planned to use the truck for more than occasional towing so I never put one in. I'm @$$uming the trans temp increased substantially along with the engine temp. But towing in O.D. locked up things stayed pretty cool. Could even run the A/C.
     
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  2. HotRodPC

    HotRodPC Administrator Staff Member

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    See, alot of people will tell you NOT to tow in OD. I'll call bullshit on that. Personally, I think if you have the correct gear ratio to take most of the strain off the transmission, converter is locked up to help keep it cool, then why not? Now dont be having no 3.08's and towing in OD, or 3.73's and pulling 10,000lbs.

    You can also get a Super Hold 4th Servo that'll hold the 2/4 band tighter than stock. That should be some added help too when towing. Also get the Kevlar wider 2/4 band so it's holding more of the drum. I also use the Alto Commercial 3/4 clutch pack that tolerates a bit more heat. And with an added aux cooler, I see no reason not to if you have the correct gear ratio. Just have to consider all things.
     
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  3. shiftpro

    shiftpro Full Access Member

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  4. SirRobyn0

    SirRobyn0 Full Access Member

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    I hadn't either. When I bought the truck the wire for the 2nd / 3rd lock up was disconnected. When I reconnected it thats when I realized it would lock in 2nd. I setup the manual lock switch this morning and this is how it is setup right now. With the switch off it'll lock in 4th only. With the switch on it'll lock in 2nd, 3rd and 4th. I'll only turn it on in 3rd and we'll see how that works out on this trip.

    I think I'd tend agree, I think manufactures say no OD towing because it's safe. I've got a 1999 class C motorhome on chevrolet chassis and I drive that thing in OD a lot though I will shift to 3rd on hills. I can already know from loads I've had in the bed and what little towing I've done truck I'll need to leave it in 3rd
     
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  5. SirRobyn0

    SirRobyn0 Full Access Member

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    Turned out a little differently than I expected. The RPO codes indicate my rear diff is a 3.73 but I'm kind of having a hard time believing it's even that low. Before a regear I'll pop the cover and count. The trip over the pass was uneventful, and really the trip home was to, but as it turned out the small tractor I picked up weighs in at 3,000lb not 2,000 as I had thought. So total 5K gross on the trailer.

    On the steeper hills and over the pass on the way home I spent a good bit of time in second gear, but maintained 60MPH even over the steepest part and second gear was not to high of RPM (no tach), but I know engine speed pretty well from just being around for so long. 2nd gear at 60MPH felt like it was right in the power band. So either I've had a lot of low geared rigs in the past or this truck is higher geared than I thought.

    @HotRodPC, I had thought about setting up a manual switch to run the converter clutch in 3rd, but never did, so thank you for bringing up the whole locking in 3rd thing which reminded me to do it. I used it quite a bit for sure on the trip and it did make a noticeable difference in temps when I had it engaged. I was even able to lock it in 2nd on some of the hills when I had to drop into 2nd, so I finally found a use for 2nd gear locking.
     
    Last edited: Jun 28, 2020
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  6. HotRodPC

    HotRodPC Administrator Staff Member

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    You'd have been turning 4000 rpms with 3.73's and about 3600 with 3.42s in 2nd gear. So it must be a tall gear. It's a 14b Semi Float right? I know 3.42's did exist cuz I have one that came from an 85 C20, also 3.73 and 4.10.
     
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  7. SirRobyn0

    SirRobyn0 Full Access Member

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    Yes it's a 14b semi float. I do not believe that I was running 4,000 RPM especially since peak HP is listed at 3800RPM, 3,600RPM sounds believable, though I would have guessed a little lower. I think it's time to install a tach and count some teeth.
     
  8. SirRobyn0

    SirRobyn0 Full Access Member

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    Well despite my RPO code indicating 3.73 gears, it's got 3.42 gears in it, I'm guessing that means the differential was swapped at some point in it's life. So a re-gear is going to happen very soon, that's just to tall a gear for my needs. So the question is, any preferred vendors? I've been thinking of ordering from East Coast Gear.

    I've fiddled around with a rpm / ratio calculator and I'm pretty well set on the 4.56 gears I think. But 4.10 and 4.88 gears are available.

    Keep in mind there are mountain passes, immediately to the east of me and to the south of me, even going in the other directions where there aren't passes there are hills everywhere. My point is ability to climb these hills with the truck loaded and / or towing 5 - 7K as best as possible is very important to me. Obviously I know it's a stock 350 so I'm only going to get so much, but I want to do what I can with a re-gear. At one time I had an RV with a dodge 360 in it, and even towing a car it did better than the truck on the hills towing, so I really think re-gearing the rear end could make a big difference. Anyhow I wanted to open up for opinions on this. Keep in mind I have the 700R4 so I do have over drive for when I'm at higher speeds and not loaded.
     
  9. AuroraGirl

    AuroraGirl Supporting Member Supporting Member

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    to my understanding, not using OD for towing is more about load, yes? IF expected to need to downshift or to go up, down, etc its best not to?

    my f150 says its to keep the trans cooler by not up and downshifting constantly, but if you are on flat ground it shouldnt do that
     
  10. SirRobyn0

    SirRobyn0 Full Access Member

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    The only way we'd know for sure is if a tranny engineer were to tell us. But like I said in another post with my motorhome I can definitely drive it in over drive. I don't know the weight off hand but it's a class c so its a van chassis, 32 feet. I think the biggest reason is to reduce shifting, and to keep the converter locked up as much as possible. I just try to use logic, if I'm loaded or towing and OD causes lugging or excessive shifting I take it out of overdrive.
     
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  11. HotRodPC

    HotRodPC Administrator Staff Member

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    4.56 would probably do nicely if you're running the stock 31 in tall tires.
     
  12. SirRobyn0

    SirRobyn0 Full Access Member

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    Yea it's the stock 31" tall tires. I'll be putting in the order for the gears on Monday and then it'll be time for me to brush up on setting up the gears, it's been so long, we send out most of the differential work at the shop these days.
     
  13. AuroraGirl

    AuroraGirl Supporting Member Supporting Member

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    Surprising that something so simple to you or I is perplexing to others.
     
  14. shiftpro

    shiftpro Full Access Member

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    Like Dysco flapping his stupid mouth off over in the politics section haha.
     
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  15. SirRobyn0

    SirRobyn0 Full Access Member

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    Very true, even more these days the average person does not know squat about their car, but in 1984 overdrive autos were very new, so I think that just keeping it in drive is playing it safe from a manufacturing and warranty prospective.
     
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