Converting Low Speed Transmission To Taller Gearing?

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dieseltoy
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Converting Low Speed Transmission To Taller Gearing?

Post by dieseltoy » Mon Feb 05, 2018 6:29 pm

I have a 74 710M with the low speed transmission, and would like to have a little taller gearing so I can keep up with traffic.
Unfortunately these transmissions seem to be fairly expensive, and I was thinking about changing the gearing in mine instead of spending a pile on another one.
Are any of the other ZF transmission models suitable donors for the taller gears I am looking for?
Or, does anyone have a lead on an available high speed transmission?

Thanks for any help,
Ed

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Jimm391730
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Re: Converting Low Speed Transmission To Taller Gearing?

Post by Jimm391730 » Mon Feb 05, 2018 6:36 pm

How fast do you want to go to "keep up with traffic"? I can cruise at 60-65 all day (in warm temperatures, when the gearboxes get warmed up) in our stock 710M. But I'd prefer to be the slow traffic in the right hand lane to ensure there is plenty of gap between us and the traffic ahead of us...
Jim M.
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dieseltoy
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Re: Converting Low Speed Transmission To Taller Gearing?

Post by dieseltoy » Mon Feb 05, 2018 7:36 pm

60-65 would be great. I am currently limited to ~35mph in 5th gear with 255/85 tires

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Re: Converting Low Speed Transmission To Taller Gearing?

Post by Jimm391730 » Mon Feb 05, 2018 7:48 pm

I am currently limited to ~35mph in 5th gear with 255/85 tires
No, you are not. Your tires are 4.5% larger in diameter than the 265/75R16s that I run, so you should be able to go 4.5% faster... ULESS YOU ARE IN LOW RANGE! The transfer case lever is above the gas pedal on the side of the air tunnel; forwards is high range, back a notch is neutral, and pulled towards you is low range. Low range is not quite 2:1 (actually 1.92:1) so you can't quite go double that speed in high range which is why I'm thinking you are stuck in low!
Jim M.
712W and 710M

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dieseltoy
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Re: Converting Low Speed Transmission To Taller Gearing?

Post by dieseltoy » Mon Feb 05, 2018 8:04 pm

I am familiar with the operation of the transfer case, and it is in high range. Low range is considerably slower.
Just out of curiosity, does anyone know what the gear ratios are for the "off-road" transmission?

Thanks

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Jimm391730
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Re: Converting Low Speed Transmission To Taller Gearing?

Post by Jimm391730 » Mon Feb 05, 2018 8:11 pm

There is only one transmission for the 712-710 "gassers"; a few of us have traded them for the overdrive Turbo Diesel 718-716 models (gains "6th" gear, loses 1st). But that won't gain you but 39% higher speeds (from 35mph up to 48mph). You either aren't getting the engine revs up or don't have the transfer case in high range. I also won't get past ~35mph if I forget and leave it in low when I get off a trail which is why I suggested it... your max speed is a virtual match for low range.

There have been a couple of trucks that had underdrives custom installed between the transmission and the transfer case. What truck do you have, who did you get it from? 35mph is NOT right...
Jim M.
712W and 710M

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David Dunn
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Re: Converting Low Speed Transmission To Taller Gearing?

Post by David Dunn » Mon Feb 05, 2018 8:32 pm

Jimm391730 wrote:
. Low range is not quite 2:1 (actually 1.92:1) so you can't quite go double that speed in high range which is why I'm thinking you are stuck in low!
The transfer case is .88 in high and 1,69 in low for the 710's
The 712's are 1 to 1 in high and 1.92 in low

Dieseltoy, a 710 will do in excess of 70 mph with 255/85 tires.
My first thought would be that you are in low range..... if you are sure you are in high range,try shifting into low range and see what kind of speeds you can do. Also if the shift lever moves easily , you might have a broken shifter cable
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dieseltoy
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Re: Converting Low Speed Transmission To Taller Gearing?

Post by dieseltoy » Mon Feb 05, 2018 8:37 pm

And here I thought I had correctly diagnosed the issue. I assumed that there were two transmission variants. The factory service manual (p50) describes the top speed for the off-road transmission as 54 km/h.

No underdrive is installed.

It runs good, shifts through all the gears, and high/low range, f/r lockers all work.

I purchased the truck from a private party who seemed to know very little about it. I can just make out from the paperwork that it was imported in 2011, and has been driven approximately 1500 miles since then.

Thanks for your help.

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dieseltoy
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Re: Converting Low Speed Transmission To Taller Gearing?

Post by dieseltoy » Mon Feb 05, 2018 9:03 pm

David Dunn wrote:
Jimm391730 wrote:
.
My first thought would be that you are in low range..... if you are sure you are in high range,try shifting into low range and see what kind of speeds you can do.
That was my first thought as well, but low range (pulled back towards the seat) is definitely lower gearing and considerably slower than high range (lever pushed all the way forward)

Thanks for your help!

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Re: Converting Low Speed Transmission To Taller Gearing?

Post by Jimm391730 » Mon Feb 05, 2018 10:17 pm

I assumed that there were two transmission variants. The factory service manual (p50) describes the top speed for the off-road transmission as 54 km/h.
The "off road transmission" in the manual means that the transfer case is shifted into low range. It is not a separate or different unit, it is that the transmission of power is going through further reduction by way of the low range in the transfer case. This is the "off road" power transmission path. It's the way it was translated into English.
Jim M.
712W and 710M

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Jimm391730
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Re: Converting Low Speed Transmission To Taller Gearing?

Post by Jimm391730 » Mon Feb 05, 2018 10:26 pm

On the other hand, I've met owners that refused to run with rpms beyond maybe 3000 rpm because they didn't like the "sound" of the fan and engine (40-45mph tops). Not that it is bad to keep the revs down, but these engines (if well maintained) don't mind running close to the 4500 rpm redline for hours at a time (I've done it on cross country highway trips). If you aren't sure, put a tach on it to see what rpms you are at when going 35 mph. You should be able to do 60 mph at 3700-3800 rpms in high range and the stock 5th gear with the tires you've got.
Jim M.
712W and 710M

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Re: Converting Low Speed Transmission To Taller Gearing?

Post by VinceAtReal4x4s » Mon Feb 05, 2018 10:54 pm

Post some pics of the trans set-up, driveshaft area, number plate, etc.
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Re: Converting Low Speed Transmission To Taller Gearing?

Post by edzz » Mon Feb 05, 2018 11:16 pm

You have not mentioned where you are located. Please edit your profile to include your location, you may find another forum member that is close to your location that could help diagnose your issues in person.

Ed
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Hotzenplotz
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Re: Converting Low Speed Transmission To Taller Gearing?

Post by Hotzenplotz » Tue Feb 06, 2018 3:20 am

lads

let me contribute the gear ratios

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The 5 speed gearbox is a ZF S5-18/3 used in several vehicles of the 70 and 80ies.
Maybe you can find a box that fits your needs.
Cheers
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TechMOGogy
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Re: Converting Low Speed Transmission To Taller Gearing?

Post by TechMOGogy » Tue Feb 06, 2018 7:40 am

Another to vote - either something is seriously wrong with your transmission, your in low or your running it with really low rpm.
I have 255/85R16's and can easily go 60-65 mph and thats with rpm around 3500. I have had it up to 70mph just to say I did it but that was just on a short stretch and I was not overly comfortable at that speed :)
72 Pathfinder | 75 710M 2.7i | 96 350GDT Worker

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