[bozzaa]

Can anyone tell me if a sierra w/t with 31's, stock transfer but 5.12 diff gears is goi g to get it somewhat close to stock gearing? I have the gears sitting there but trying to see if its worth throwing them In for a trip I have coming up.

Cheers.

[SierraDan]

Appsie used to have 5.12 with 31's, he said it did the job.

[bozzaa]

Perfect

[Gwagensteve]

It's way less than ideal. Revs will be around 3641 at 100 by my quick number crunching ( I'll confirm tonight when I'm at home) but low range will still suck for anything steep/technical. The problem is, if you hate the low range and want to do transfer gears, road rpm goes sky high. 4200rpm+

Steve

[bozzaa]

Will do transfer gears eventually Steve and bigger tyres. Just getting her ready for vic zook club June 2nd trip. Honestly road rpm I ain't too fussed about. It isn't a daily thank god. Everything i do ends up un-finished for that day.

[Gwagensteve]

In my opinion, you'd be better to fit 6.5 transfer gears first up. Fitting 5.12 diffs is expensive, doubly so if you need to pull them out later for lockers/ cr-mo axles etc.

On top of that, most guys in the club are happy with stock diffs and 6.5 transfer, even with 34's. revs are a bit low on highway, but its no big deal.

Just look carefully at the costs. Very few people need 5.12's on top of transfer gears.

Steve

[bozzaa]

Thanks Steve. Got gears already and mate is putting them in for me. Will keep what you have said in mind though if all gets too hard!

[christover1]

Will be a lot better than stock.
If fitting is free, I say go for it.
Is easy job to swap back when tranny gears are done, if needed.

[Gwagensteve]

OK, at home.

Here is a comparison between a stock sierra, 5.12's with 31's, and 6.5 transfer gears only, and 6.5 gears with 5.12's with 34" tyres.

Stock sierra on 26" tyres

tyre diameter in inches 26.00inches
1st gear 3.65 :1
top gear ratio 0.83 :1
Transfer case high range ratio 1.41 :1
Transfer case low range ratio 2.21 :1
Diff ratio 3.70 :1
revs @ idle 750.00 rpm
revs @ peak torque 3500.00 rpm

Desired cruising RPM 3500.00 rpm
Speed in km/h 100.70 km/h

On road data
Cruise RPM @ speed
Desired speed in km/h 100.00 km/h
rpm in top gear@ 100km/h 3475.57 rpm
Off Road Data
Crawl ratio 29.85:1
speed@idle in 1st gear 3.13 km/h
speed @peak torque in 1st gear 14.60km/h

5.12's with 31's

Tyre diameter in inches 31.00inches
1st gear 3.65 :1
top gear ratio 0.83 :1
Transfer case high range ratio 1.41 :1
Transfer case low range ratio 2.21 :1
Diff ratio 5.12 :1
revs @ idle 750.00 rpm
revs @ peak torque 3500.00 rpm

On road data
rpm in top gear@ 100km/h 4033.72 rpm
Off Road Data
Crawl ratio 41.30 :1
speed@idle in 1st gear 2.70 km/h
speed @peak torque in 1st gear 12.58 km/h

31's with 6.5 transfer gears

tyre diameter in inches 31.00 inches
1st gear 3.65:1
top gear ratio 0.83:1
Transfer case high range ratio 1.68 :1
Transfer case low range ratio 6.50 :1
Diff ratio 3.70 :1
revs @ idle 750.00 rpm
revs @ peak torque 3500.00 rpm

Desired cruising RPM 3500.00 rpm
Speed in km/h 100.70 km/h

On road data
Cruise RPM @ speed
Desired speed in km/h 100.00 km/h
rpm in top gear@ 100km/h 3475.65 rpm
Off Road Data
Crawl ratio 87.78 :1
speed@idle in 1st gear 1.27 km/h
speed @peak torque in 1st gear 5.92 km/h

34's, 5.12 diffs and 6.5 transfer:

tyre diameter in inches 34.00 inches
1st gear 3.65 :1
top gear ratio 0.83 :1
Transfer case high range ratio 1.68 :1
Transfer case low range ratio 6.50 :1
Diff ratio 5.12 :1
revs @ idle 750.00 rpm
revs @ peak torque 3500.00 rpm

Desired cruising RPM 3500.00 rpm
Speed in km/h 79.81km/h

On road data
Cruise RPM @ speed
Desired speed in km/h 100.00 km/h
rpm in top gear@ 100km/h 4385.18 rpm
Off Road Data
Crawl ratio 121.47 :1
speed@idle in 1st gear 1.01km/h
speed @peak torque in 1st gear 4.69 km/h

Whilst some of these figures are a bit loose, for the purpose of comparison, they're telling.

5.12's increase cruise RPM a lot - they are far lower than is required to correct the added tyres diameter.
However, off road, they only just compensate for the tyre size increase - however, 31's offer much more traction than a stock tyre, and are heavier, so in effect, the car might not feel as well geared as stock.

I call that a loose-loose - road use is compromised for very little increase in off road performance.

Compare that to 6.5 transfer gears - the 6.5's basically correct 31's near enough exactly on road, but offer over double the low range reduction of 5.12 diffs. That's a win-win.

You can also see the big problem with wanting to add transfer gears later if you step up to bigger tyres. Throwing 34's into the mix with 5.12's and 6.5 gears, you'll see that the car is getting pretty undriveable on road - 4300rpm @ 100 is going to get old really quick, but the 121:1 first low is a pain with a carby sierra motor unless you're keen for super-technical crawling - you just run out of low range too quickly.

By all means try the 5.12's, but I'm prepared to put money on you wanting to swap to some other combination of transfer and diffs later on, even if you trade up to much larger tyres.

To provide some context, I've run 34's on a 1.0 with just 6.5 transfer gears, (4:1 diffs) a 660 turbo with the same tyres and gears, then the 660 with 5.12's and 6.5 transfer, and now an AW-4 with 35's, 6.5 transfer, and 5.12 diffs.

Steve.

[bozzaa]

Thanks for that Steve, and thanks for putting that so practically for dummies like myself.
Based on that data and what you have said, I am sort of spewing as the biggest tyre the car will see is a 34", so i really shouldn't require the 5.12s at all. Although, as I never really drive over 80 in the Sierra anyway as I like to be able to hear what the person next to me is saying, I truly believe the on road performance would not upset me all that much, just the truckers sitting on 110 behind me to get up that next hill.

But......I am going to put them in for a couple off reasons. They are sitting there, and I want to learn how to do some things. something that can probably be calculated as ignorance.

Thanks again.

[Gwagensteve]

No worries- happy to help.

Steve

[86slowsierra]

i ran 5.12's and a 1ltr case, on 31's and 32's for a couple of years and was quite happy with them. like you i had them laying about so in they went.
yeah 100km was about the 4k mark but it did it good enough. and low range was, well, low enough.
Obviously tcase gears is the right way to but from my personal experience 5.12's did the job.

[elky]

86slowsierra wrote:

i ran 5.12's and a 1ltr case, on 31's and 32's for a couple of years and was quite happy with them. like you i had them laying about so in they went.
yeah 100km was about the 4k mark but it did it good enough. and low range was, well, low enough.
Obviously tcase gears is the right way to but from my personal experience 5.12's did the job.

yea i wouldnt get 5.1's specifically, but i had braced diffs with a autolocker here so in they go, transfer gears are a much better option, and one i will be doing after the other stuff is done

[bozzaa]

Though I would bump this for future reference. Gears are in, used vitara alloy centres, changed out sierra side gears for the rear and re-shimmed. Front bolted straight up. She works hard on the road, but worked a treat I found for off road use. Prob not too good for a daily.

[bozzaa]

Though I would bump this for future reference. Gears are in, used vitara alloy centres, changed out sierra side gears for the rear and re-shimmed. Front bolted straight up. She works hard on the road, but worked a treat I found for off road use. Prob not too good for a daily. I only paid $100 for the set if gears, so I am glad I did it.

[joeblow]

Did you leave them in the alloy housings or swap them over to the steel Sierra jobs?

[Fatzook]

joeblow wrote:

Did you leave them in the alloy housings or swap them over to the steel Sierra jobs?

Sounds like he kept the alloy housings to me

[bozzaa]

Ally for now. They will be going in steel when I put lockers and cromos in. Will leave all that for a diff builder I think.

[Gwagensteve]

I know that Skyman ran alloy housings for some time without apparent issue. It's also worth noting that 660cc Jimnys run alloy front housings.

I wouldn't do it though, because I think it's false economy. If the housing fails its likely to wipe the gears out as it goes.

Steve.

[christover1]

The forces on the casing by a Sierra solid axle may differ from the rather rigid Vitara front end.
You may be lucky, time will tell.

[Gwagensteve]

They 100% are different, any you may be lucky, but that's not to say they aren't going to break (especially the rear) and the gears will be damaged as the housing fails.

Steve

[bozzaa]

Thanks for the input guys. I do have a spare in the shed and its only a temp solution to get the Sierra out of the shed.