4.12 gearing on 235/85R16

outono

Well-known member
Sep 26, 2020
162
85
Orange, CA
Im prepping to do some work on my diffs and I figured I ought to give some consideration to a re-gear while im in there. The plan at the moment is to toss some Ashcroft ATBs front and rear, along with 4.12s to give my vehicle the extra kick it needs up long inclines, especially when its loaded up with equipment and front / rear bumpers.

I know 235/85 tires don't really need a re-gear for street driving (truck drives just fine right now), but I want to get some opinions from those that have gone this route. Do you feel the investment worth it? Is the engine noise at the higher RPMs bothersome? How much of a hit are you taking on mpg?

I wrote to Ashcroft and Dave's reply is that I would be best served going with 4.12, esp. with a D1 that has heavy bumpers and a rack.

On a side note: no, I don't want lockers. I've got another vehicle with factory front and rear lockers so i'd really like to do something different on this Rover.
 

lunchbox

Well-known member
Apr 20, 2004
2,133
165
50
St Louis, MO
One of the trucks I built has 4:11s with 35s and the speedometer is perfect. The tach is where it needs to be at all speeds as well. You’ll be pretty high in the RPMs on the highway with that combo. Nothing will explode. But it may be really annoying.
 
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outono

Well-known member
Sep 26, 2020
162
85
Orange, CA
Ah ya thats my main concern is just how annoying it might be. I feel like 3.75 isn't worth it, but maybe it is?
 

Flyfish

Well-known member
Oct 29, 2004
1,402
212
52
St. Louis
give my vehicle the extra kick it needs up long inclines, especially when its loaded up with equipment and front / rear bumpers.
I’d imagine that’s gonna cost a lot of money for such a small modification. Depending on cost, maybe it’s worth considering a 4.6. It’s really not an expensive swap if you’re doing the work yourself. Should be less than $2k which is probably the same as ATB’s and gears. Just a thought.
 
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LRDONE

Well-known member
Dec 3, 2020
336
116
Michigan
I’d imagine that’s gonna cost a lot of money for such a small modification. Depending on cost, maybe it’s worth considering a 4.6. It’s really not an expensive swap if you’re doing the work yourself. Should be less than $2k which is probably the same as ATB’s and gears. Just a thought.
Flyfish, do you still have that 4.6 in your garage? Ready to sell??
 

ERover82

Well-known member
Nov 26, 2011
3,899
450
Darien Gap
That's a good point. Even with correct gearing, these things are still underpowered by today's expectations. Ideally, you do both, but that's a lot of time and cash.
 
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Flyfish

Well-known member
Oct 29, 2004
1,402
212
52
St. Louis
That's a good point. Even with correct gearing, these things are still underpowered by today's expectations. Ideally, you do both, but that's a lot of time and cash.
Exactly. Definitely get more improvement per dollar doing a 4.6 vs gears. And I wouldn’t re-gear until you change to 33”+ tires.
 

K-rover

Well-known member
Jan 15, 2010
2,163
62
Raleigh, NC
I also run 4:11#s with 35son my D2 and its pretty good. Runs about 2750 rpm at 70mph. It would be nice to have a little bit lower gear offroad, but its great on the road.
 
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Flyfish

Well-known member
Oct 29, 2004
1,402
212
52
St. Louis
Let me know when you're ready to part. I've been considering the 4.6 for awhile now.
That was gonna be sent to a machine shop for top hats etc and go into the RRC. But at this point, it’s just giving my engine stand something to do. We can discuss it whenever you start looking. You have my number and can shoot me a text.
 
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jymmiejamz

Well-known member
Dec 5, 2004
6,008
361
35
Los Angeles, Ca
I think 4.12 will be too low. I felt like the 4.11s I had in my D90 with 8.25R16 Michelin XZLs was too low.

A 1.4 ratio transfer case would probably be a better option
 

Gordo

Well-known member
id suspect the 1.4 box will be really close to 4.11 but i could be wrong. Weve got one in our D1 on 33s and it does work well with that size. Probably the cheapest option but unfortunately its only high range and with our 5spd that isnt ideal, especially in the rocks, luckily for us were in FL so its more mud. We did it because the cost was much cheaper than gears. I got the box for 200$ from a buddy. The truck is noticeably more peppy with the 33s and 1.4 when compared to stock gears and 235s.
 

outono

Well-known member
Sep 26, 2020
162
85
Orange, CA
OK so after some testing, im wondering if this vehicle is already re-geared or has a 1.4 box.

Using my ScanGauge, im seeing ~2667 RPMs at 70mph. The Ashcroft calculator says that's about 4.11 gearing OR a 1.4 TC with 3.54 in the diffs (on 235/85 tires). This is a bit of a surprise to me and maybe im overlooking something. If the vehicle had stock gearing with my 32" tires, I should be at ~2318 RPMs at 70mph.

How can I check what im running without pulling the gears?