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Discussion Starter #1
Ok, I need some edumacation here.

Turns out my pinion bearing took a shiat. Damn shaft is wobbling in there. So... Since I have to get in there again, I am thinking of re-gearing.

I have heard that I shouldn't run series 4.70's, but here's my question:

Are 4.70's too high for a highway driven rig on 33x11.50's?

I need to be able to cruise at 75mph, and handle the 13k-14k passes without flogging the beast. I have a mild cam, just a K&N and am running about 3.5lb on backpressure, (center muffler only). I know that most people run 4.10, but I am just curious. I do mostly around town in it for the most part, but It does see several 1200 mile trips in a year. Oh yeah, detroit rear and tru-trac front. I have given up on fuel economy.

j
 

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3.9 right?

Well, here's my experience.

I have one of the LWB's (4.2) set up with 4:11's, the other one is still running the 3:54's). Both have 235/85/16's (Dunlop Radial Rover R/T's.

Personally I think a gearset that's halfway between 4:11's and 3:54's is ideal with that tire size and engine combo. If I'm hauling a heavy load I'll drive the LWB with the 4:11's, if I'm cruising to SLC I'll drive the 3:54's.

At 75-80 mph I'm turning at just over 3K with the 4:11's.

With the little smaller motor, and perhaps even a little more tired than my 4.2's (both have just over 110K), you might want to go for the 4:11's.

With your 33 inch tire size you maybe ought to stick with the 4:11's, I don't know what kind of difference the 4:7's with the 33's would do to the RPM's at speed, but again, if you have a motor that's a bit tired I would think that taking it a little easier on it might be smart???

'Course I dunno what your ultimate plans are with the engine, if you just want to run it till it's dead, or if you even care about longevity, or what. Not saying that you're stupid, because I feel like I'm talking over my head a little here. That's why I'm trying to relate it to my experience with my trucks.

You might want to ask Ali's opinion. He was thinking the same thing and took the two different RR's I have out for a test drive, one after the other, while he was here visiting.

It's the dilemma's like this that you may not know the best course of action until you choose one and try it out. Hopefully you choose what you need/want.

I'm curious how it turns out, so get crackin'!
 

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3000rpm with 3.54s and 33s= 93mph

3000rpm with 4.11s and 33s= 80mph

3000rpm with 4.70s and 33s= 70mph

and for reference

3000rpm with 3.54s and 29s= 82mph

so 4.11s would but you back into a stock power curve on the highway.

have you considered just sticking with the 3.54s? if just your rear pinion bearing crapped out then you should just have to rebuild one diff, not even neccessarily replace the gears. but it you changed gears you will end up spending $1000 just in parts for the gears.
 

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Discussion Starter #4
Yeah I have discussed it, and think hard about it. I have to get in there on at least one, so you know, while your there...

Ashcroft is under 200 plus freight plus the flange per diff.

You guys also forget just how anemic a 3.9 can be at 13,000 feet.

I think I will just stay the same and see how it is with the new tires.

I am putting them on today. (whohoo!) I just think 4.10's might be the ticket, but I want some more opinions....

j
 

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redrangie said:

You guys also forget just how anemic a 3.9 can be at 13,000 feet.

j
Oh no, I haven't forgotten. I remember quite well what the '90 was like before the 4.2. :(

When I did my 4:11's I got them from GBR. I dunno if they were the ashcrofts or not. I seem to remember they were about $400 a pair (is that right???).

At $200/pr. I'd do the r&p and not think twice (except about the ratio that is....).
 

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redrangie said:
Ok, I need some edumacation here.

Turns out my pinion bearing took a shiat. Damn shaft is wobbling in there. So... Since I have to get in there again, I am thinking of re-gearing.

I have heard that I shouldn't run series 4.70's, but here's my question:

Are 4.70's too high for a highway driven rig on 33x11.50's?

j
Don't go with the 4.7's, those are series LR R&P and are weak!
4.75's are hopefully what you mean?
They are stronger, will give you better power for the mountains but at the penalty of higher rpms ( as others have pointed out )

anyways just don't do 4.7's do 4.75's if you want that kind of ratio ( I poofed the first set ever installed on a Disco in the US about 7 years ago :D )
 

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You guys also forget just how anemic a 3.9 can be at 13,000 feet.
At that altitude I would go with the 4.75s, especially since you are doing mostly around town work.

As others have pointed out, the 4.11s only get you back to stock 3.9l rpm vs speed with 33s. Above 12K you are looking at something like 70% of stock 3.9l power which blows bigtime for hill/mountain climbing.

I run 285s with 4.75s and am happy. But I really think 4.5s (if they existed) would be the ideal solution. But then I spend most of my time driving up and down extinct (hopefully) volcanos in Central America.:D
 

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Busto said:
At that altitude I would go with the 4.75s, especially since you are doing mostly around town work.

As others have pointed out, the 4.11s only get you back to stock 3.9l rpm vs speed with 33s. Above 12K you are looking at something like 70% of stock 3.9l power which blows bigtime for hill/mountain climbing.

I run 285s with 4.75s and am happy. But I really think 4.5s (if they existed) would be the ideal solution. But then I spend most of my time driving up and down extinct (hopefully) volcanos in Central America.:D
i disagree. i would NOT want the 4.75s at 13k ft because the engine would be working in an RPM range where it does not make as much torque. the 4.11s with 33s would be an excellent choice IMO because it will keep the engine right in its torque curve at highway speeds. which is better for climbing mountains.
 

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9V said:


i disagree. i would NOT want the 4.75s at 13k ft because the engine would be working in an RPM range where it does not make as much torque. the 4.11s with 33s would be an excellent choice IMO because it will keep the engine right in its torque curve at highway speeds. which is better for climbing mountains.
So off the cuff, I believe the power band is somewhere between 2k & 3k rpms... We have property in the Colorado Mtn's and I have been there with:

Series II
stock Disco
Disco with 4.7's and then 4.75's
D90 with blower and 4.75's
all with tire sizes ranging from stock to 35's...
Here is what I have to say about power band...

ON big mountain roads the only place my foot was... on any of the vehicles was firmly planted on the gas pedal, denting the floorboard...

None of the vehicles would ever run the hills at nice 2-3k...
They were winding in 3rd even 2nd....

The 4.75's made a big difference, obviously the blower made an even bigger difference... but the point to this is:

its not about power band, its about if you want to run at 3200-3500 rpms on FLAT higways at 7X mph all the time ( IE: wear )
its obvoius the higher the ratio ( ie: 4.75's ) will do better on the hills.
 

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Just as an aside, has anyone installed a set of 3.80 r&p? For Slade, and myself, this sounds like a good compromise given our 32" tire sizes.

Sorry, John, didn't mean to sidetrack the topic here :p

Ali
 

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or if you like to travel faster then with 4.11s you are in the torque range at 80mph
Look at the torque curve from RPI - pretty flat to almost 4 grand. That is one of the good points about the rover V8s, they have reasonably flat torque curves through most of the rpm range. The 4.75s will still keep the 3.9l very much in its powerband at 80 mph. At 80 mph the motor will be turning at about rpm with 4.75 gears and at that rpm we are looking just 290 Nm of torque (92.9% of max torque).
 

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4.11's work very well on my overweight Disco (too much? extra steel) with 32's.. I think i woud go to a deeper t case gearset if i was going to go lower in the final drive... Like a Diesel LT230 gearset.
 

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alia176 said:
Just as an aside, has anyone installed a set of 3.80 r&p?
Ali
Apparently Santana made a R&P that were close to 3.80 Ali. I was talking to Timm about it one day and he actually knew about them. Maybe I'll try to track down a set and install them in the other LWB.

Back to your regularly scheduled thread...


-end hijack here-
 

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Discussion Starter #19
Ok,

I did mean 4.75's. So sue me. Most of the Toy people run 5.29's around here for the altitude. (even the supercharged ones)

On 3.54's with 32's, on flat road, say around grand junction to green river, under load, I will be easily doing 3000 to maintain 75mph IF and that's a big if I can keep it out of third. Hit any hill and I am in 2nd doing 4400.

I mean come awn. 75 mph is fast enough for anyone in a holey bushed, 5" lifted, no sway bar'd sled, non - castor corrected sled. Right?

j
 

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$200 a pair?? :eek: I looked at Ashcroft's website, and it said that the 4.11s were 240 Pounds per pair, and that works out to be around $400 per axle. I really hope i am missing something here.:confused:

Brad
 
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