gearing

[85_sierra]

just about to rebuild my transfer case in my sierra and i was think while it is open should i get some lower ratio gears. the car has a 20v 4age, 5.12 diff gears and 31inch tires. the car weill get driven on just about everything from the beach to rocks in the bush. it all ready revs hard at 100km about 4000 rmp so i dont wont to make that any worse if i can help it. any help with this would be great thanks

cheers nathan

[v840]

If you have 5.12s and 31's you probably don't need any more gearing.

[Zook_Fan]

what sort of gearing is your gear box? if it is a standard sierra box going lower will rev that engine way to hard.

[85_sierra]

i worked out the gearing and with the new motor and box it should rev at 4050 at 100kms

[mike_nofx]

Get 33's.

[mrRocky]

i agree 33's

[Gwagensteve]

Which is why 5.12's (alone) are a terrible way of gearing a sierra.

you're spinning 4K @ 100 with 31's... and have

First Gear: 3.587:1
transfer (1.3?) 2.2:1
diff: 5.12

Crawl ratio - 40:1

That's the worst of both worlds.

If you do sand, it would rule, but in rock that's a terrible number. You really want 60:1 plus. I know you have more torque than a 1.3, so you can get by with less gearing, but if idle in first low is too fast to control the car, there's nothing you can do about it.

Have a play with a ratio converter. I think you need to aim for about 3500rpm cruise with 33's and 70:1 or so in crawl. There's about eleventy ways to achieve this, but it's unlikely to involve 5.12's unless you're in the 35" tyre region.

Steve.

[85_sierra]

well i had a go on the ratio converter and came up with this

34 inch tires
first gear 3.58
reduction 6.5:1
crawl ratio 119.14:1

but it still sits around 4100 at 100

34 inch tires
first gear 3.58
reduction 4.9:1
crawl ratio 89.82

and sits at around 4000 at 100

i can all ways just drive at 90km which brings it down a bit cause it is only a weekend car. i dont know how i am going to go trying to fit 34s because i dont wont to go spoa. it has 2inch spring and 2 inch body lift and i have cut about 30mm out of wheel arches casue of the rust it had and it has wt diffs in it so with some big offset rims it shouldent have any problem with the fronts hitting any were when steering i hope. should come pretty close to fitting i think. i dont wheel it that hard so i think the axels ans cv's should last till i can save a bit for something stronger.

what ratio do you guys think would be better because 119 is getting pretty low and how do you think the tires will go fitting

your help has been great so far thanks

[joeblow]

5.125's 'alone' are a good way of gearing a sierra, i'm not saying the best way but it is a good reliable way. zook engines are used to revving round the 4 grand mark, your tojo engine may not be. 33's would be fine for your application, bring the revs down a bit and still have good gearing, but over that size other options may have to be considered. a heap of us run 5.12's without t/case gears and find them great, and if we want a little extra we will go auto before t/case. but it all comes down to your intensions as far as tyre size, and what you end up driving the most.

[Gwagensteve]

I don't think you need 119:1 crawl.

In fact, I don't think you're going to really need 89:1 either if you don't really wheel it that hard.

Personally, I'd go to a taller diff gear and run 4.9 in the transfer.

I'm not familiar with the powerband on a 20V, but I bet it was only spinning about 3K @ 100 in the corolla it came out of.

Steve.