I've got 35's with 3.73 in a 6 speed 3.6L and it takes a lot of clutch on the trail. City it is alright, you just need a little extra throttle off the light but freeway the sweet spot is between fifth and six. Mine is pretty much dead on with the chart as far as 70 mph at 2000 rpm. Backing up the driveway is a pain cause it either wants to stall or run too fast so again I'm riding half the clutch. It's acceptable but not ideal. Climbing it is pretty bad but everyone is getting used to the smell of my clutch.

The newer manual trans rigs have a crawl mode where the Jeep will maintain enough throttle to keep it from stalling so you can adjust speed with the brake and stay off the throttle. I can't use it anymore cause it just stalls on any incline or rock.

I'm getting 4.88's very soon. If it was a 3.8 I would probably be getting 5.13's but I do a lot of crawling. I'm not sure what the d30 can handle though before you start twisting parts off.

 

Just curious ...are you running manual hubs with the Detroit up front?

 

No will be bolting on all my stuff when all my parts arrive. Dynatrac told me it will be invisible on road in 2hi with the new soft locker. And it will be a locked beast off road when I throw it in 4. It's not for everyone but the terrain we have here in the south is fine with a locked front end. All my friends run detroits and love them. No air lines or wires to break. WOL seems to like ARB which I think is due to his terrain and extreme wheeling. With that kind of offloading I can deffinetly see where you would want a selectable. But I'll be mostly in mud sand and light obstacles.

 

Sounds good. I was picturing some chirping front tires.

 

I like crawling sloooow, 4Lo is sloooooow, I was racing a bunch of ants the other day and it was pretty close! lol
Wow, I don't know how you run 35" with 3.21. Good for you though. Saves a bunch of cash.
I'm still on the 32" with my 4.88 lol BEAST! Slow but BEAST! I'm worried I'll miss the beast when I do 35" ... Sure would like to take it on a long ride, and at 55mph, it wouldn't be far but it'd be long lol

 

It performs a lot better but it is not geared for 35's. I can back up the driveway without stalling but I would be entering the garage at an unsafe speed. As far as climbing rock, again with the clutch all the way out and enough RPM to stay at idle the speed is just to fast to crawl. I could let my Jeep run but the trails we do would destroy my rig at those speeds.

I would never see properly setting up your Jeep for your application as a waste of money. Personally I see running 35' with 3.21 gearing as a waste of a Jeep. You mention her JK has open diffs, I doubt you are crawling over much.

 

The chart is of little use if you don't know what to look for. It doesn't tell you what diff gears you need. It only tells you what RPM you gonna run in 6th gear with such or such diff gears at 70mph. But what really matters is your output gear ratio, which is a function of transmission gear ratio and the diff gear ratio (plus transfer case gear ratio, but on street it is always at 1, so it doesn't really count). You can achieve the same RPM (or close) by shifting your transmission gears. This is what they are for! If all you had was 1 gear in your transmission, then yes, regearing your diffs would make a great sense.

When I say I am running same RPM on highway in 3rd gear as if I had 5.38 gears and running in OD, that means, my output gear ratio is the same in 3rd gear and 3.73 gears, as 4th gear (OD) and 5.38 gears. It is simple math:

3.73 * 1 (3rd gear ratio) = 3.73
5.38 * 0.69 (OD ratio) = 3.71

Now, for you, it's going to be a bit different since you have 3.21 gears and a different transmission.

3.21 * 1.25 (4th gear ratio) = 4.01
3.21 * 1 (5th gear ratio) = 3.21

4.55 * 0.84 (6th gear ratio) = 3.82
4.88 * 0.84 (6th gear ratio) = 4.1
5.13 * 0.84 (6th gear ratio) = 4.31
5.38 * 0.84 (6th gear ratio) = 4.52

If you look at the numbers above, you'll see that in 4th gear and 3.21's you have output ratio of about 4.88 gears in 6th gear (4.77 gears to be exact, but close enough). An auto would need 5.81 gears to match 4.88 gears on a manual. Your mistake is you think that since auto has 3.73 gears and your manual has 3.21 gears, then auto is somehow better. But it is NOT. Your transmission more than makes up for the higher diff gears. And we're talking here about 33's for crying out loud! Most people don't even feel the need to regear with 33's. 35's work out great with stock gearing if you know what you're doing. I can't comment on 37's and up, but experimentation and careful analysis will give you the right answer. In any case, all you'll be doing is raising your RPM to unleash the power of the engine, but you're not adding any new power that wasn't there.

I don't need to drive it to tell you what output gear ratio is. It's all math.


Some say it ain't ideal with 4.10 gears and stock tires. So? It's a personal taste. The Jeep is not a rocket and I don't expect it to be. Seems like you do though, that's why I said you bought the wrong vehicle. Buy a Corvette next time.


My butt is in a higher geared vehicle with bigger tires, and you don't see me crying.

You're arguing against math. Good luck.

 

You have valid points to regear, since it looks like for you 1st gear is unusable. I'm just wondering why for others it is totally fine

 

Well sounds like you need to get the calculator back out Bill Nye. Bc you got a lot of people telling u 321 sucks.

Guess this is one of those instances where a guy sitting at his desk with pen and paper wants to argue with the people who are actually out in the real world mashing the clutch on a jeep with 321.

 

Didn't know math is so hard for you to digest. Then feel free to do whatever you want.