[kuta]

hello all looking for some of your Knowledge

looking for the best way to get reduction for 31's or 33's down the track. thinking the 4.57 or 5.38 Crown Ring and Pinion Gears off low range as i would like to auto lock diffs and Chromoly Axle at the same time or should i forget Ring and Pinion Gears and get 6.5's? just want to do it right the first time

[norcorider]

i got 6.5s and 3.9 R&P and 31 it crawls very well and still very drivable on the road.
Also have a locked rear

[SierraDan]

I had 5.14 in the transfer and 3.9 in the diffs which I thought was good and reliable on 31's.

For bigger tyres I think the stresses of reduction should be shared better throughout diffs and transfer.

[Gwagensteve]

Diff gears are really only suitable for offsetting large tyres on road. They do nothing (significant) to improve off road capability unless they are so low you can't drive on road any more.

Here's the proof.

I've punched some numbers into my gearing calculator

Diff gears alone:

Stock sierra RPM@100: 3475
Stock sierra crawl ratio: 29.85:1 and speed at peak torque in low 1st: 14km/h

Stock sierra with 31's rpm@100: 2915
Stock sierra with 31's crawl ratio: 29.85:1 and speed at peak torque in low 1st: 17.41km/h

Sierra with 31's and 4.57 diffs, rpm @100: 3600
Sierra with 31's and 4.57 diffs, Crawl ratio: 36.86, speed at peak torque in low 1st: 14km/h

Sierra with 31's and 5.38 diffs, rpm @100: 4200
Sierra with 31's and 5.38 diffs, Crawl ratio: 43.4 speed at peak torque in low 1st: 12km/h

Yuk. with 4.57, you get back 5th gear on the highway, but you're stuck with the equivalent of stock gearing offroad.
with 5.38, you're revving too hard on the highway and it's a little bit better than stock offroad. (but only a little bit)

Now, a sierra with 6.5's and stock diffs:

Sierra with 31's and 6.5 transfer, rpm @100: 3413
Sierra with 31's and 4.57 diffs, Crawl ratio: 87.78 speed at peak torque in low 1st: 6km/h

That's more like it. Road drivability is almost the same as standard, but off road you have a proper low range.

There's no reason to go into the diffs until you are going really big with tyres. The problem is, you'll always need to gear the transfer, and as transfer gears drop high range as well, if you go to, say, 33's, you're still spinning pretty hard on the highway once you combine, say, 4.6 diff gears and a 6.5 transfer.

There have been/are advocates of using 5.12 vitara diff gears and the 1.0 transfer case, which means you can buy wrecker gears and an off-the-shelf transfer. For th sake of the exercise, I'll put those numbers in here too.

Sierra with 31's, 5.12 diffs and 1.0 transfer, rpm @100: 4437
Sierra with 31's, 5.12 diffs and 1.0 transfer, Crawl ratio: 47.65 speed at peak torque in low 1st: 11km/h

Sierra with 33's, 5.12 diffs and 1.0 transfer, rpm @100: 4168
Sierra with 33's, 5.12 diffs and 1.0 transfer, Crawl ratio: 47.65 speed at peak torque in low 1st: 11.6 km/h

I reckon that's a terrible outcome. It might be reliable because it's all stock parts and the load is shared, but in my opinion, that gearing is only good for mud once you are offroad and sucks on the highway.

Just some thoughts.

Steve

[kuta]

wow Gwagensteve you have dun your home work.
ok cool thanks for the replys i have been thinking about the best way to go for a bit. now i it will be 5.14 or 6.5's im thinking
do you have to grind the case to fit 5.14's? you do with 6.5??

[BlueSuzy]

not sure with 5.14, but yes with 6.5s, can be done with a 4"grinder. Just be gentle

[stockman]

Just get 6.5's and be done with.
Fine for 31's and anything bigger down the track.

[bogga]

had a look myself on lROR 6.5's. At the checkout if you punch in the code they give you on the home page they work out to be around $450 plus delivery. Thats a $50 saving at the moment............