Jeep Wrangler Forum banner

Biggest tires for a 3.21 gear ratio?

43K views 85 replies 36 participants last post by  failed_1 
#1 ·
Plan on lifting my jeep and want to run the biggest tires I can without regearing. Really wish for 35" or 37" but I heard I would need to regear. Anyone have any input so it can function normally on highways,offroad and onroad? Thanks guys (I assume its a 3.21 ratio gear as it is a stock 4x4 big bear jku)
 
#6 ·
A friend of mine DD'd 35" ridge grapplers 75 miles a day with an automatic for awhile before she regeared. Consensus was that it wasn't that bad. Not ideal obviously, but doable if you don't have the money to regear.

Before someone says it, your Jeep will not blow into a million pieces either and it is not unsafe.
 
#8 ·
I am running 35" General Grabber AT2's (315/75R16) on 3.21 with an auto on my 2013. Reprogrammed with a Procal. It came stock with 32s. Currently it's tolerable but when I can I'm going to upgrade to 4.56. It feels sluggish, but I don't mind.

I live in Kansas City, MO and for the most part things are flat. If it was a hillier area I'd be losing my mind.

If I finesse it on the highway I stay in overdrive. A gust of wind will knock me out. I don't usually drive above 70 anyway. My city mileage is actually better than my highway mileage. I drove from KCMO to South GA and survived - going through the mountains in KY and TN was annoying, tho.

For the record... 29" tires are what come on a base model sport. The charts basically says that the 32" tires on the Sport S are already too big for 3.21 gears, so I don't put too much stock in them.
 
#9 ·
The chart's good for determining engine speed given various tires and gear ratios, and that's about it. Ignore the colors, they're only there to sell gears.

Only you can decide what you want. Easy thing to do is downshift (which I assume you can do with an auto) at 70 to see what shorter gears feel like and do to your gas mileage. Going with a taller, heavier tire is going to hurt acceleration and efficiency. Regearing will help compensate for that a bit.
 
#12 ·
The chart's good for determining engine speed given various tires and gear ratios, and that's about it. Ignore the colors, they're only there to sell gears.

Only you can decide what you want. Easy thing to do is downshift (which I assume you can do with an auto) at 70 to see what shorter gears feel like and do to your gas mileage. Going with a taller, heavier tire is going to hurt acceleration and efficiency. Regearing will help compensate for that a bit.

Apparently the hundreds of thousands of JK owners out in the world who have been running their Jeep with stock 3.21 gears and stock 32" tires need to listen up: You need to re-gear now!..... even though your Jeep has been treating you fine for years, and even if you have no complaints, "A re-gear is in your future!"...only because the little chart posted above says you need to.

Seriously though I agree with your assessment completely but from the repeated proclamations on 3.21 gearing on this site, I'm surprised that Jeep service departments across the country aren't filled to the walls with Jeeps that folks are bringing in due to poor performance??

It sure is being presented on a near-daily basis here as a problem of epidemic proportions.
 
G
#10 ·
33s and a transmission shift point program and you will be fine. 35s would be doable but not for me, personally. there are people who drive 37 and 3.21 and don't care, i would not be able to stand that, i like the way a 33 drives on 3.21 with correct shift points.
 
#11 ·
I'm running 3.21's with 35's. Here in Florida, plenty acceptable. My off roading consists of beach driving. Otherwise, it's a tool around town toy. No complaints at all. I've got other cars that take care of my acceleration wants/needs. Guessing that I'm getting in the high 15's mpg (lie-o-meter shows 16.8 mpg ave). It runs just fine in around town stop and go traffic. I never need to mat it to hang with other vehicles pulling away from a light. If I were out in the toolies with regularly, I'd certainly want to swap the gears out. With the way I use the Jeep, it doesn't make sense. I'd never recover the cost in mpg gains, if any, or in resale down the road.
 
#14 ·
For a little while, I daily drove my JKU auto, 3.21 gears with heavy 37" tires (93 lbs each) in the worst possible conditions:

(1) at high elevation (4,300 ft - 7,000 ft typically, sometimes a fair bit higher for skiing), with corresponding power loss in our naturally aspirated engines,

(2) heavy Jeep with aftermarket bumpers, etc., and

(3) driving up, on and down large objects known as "mountains" (but no rock crawling while I had the 3.21's, just driving on roads- I'm not that crazy).

It was merely "doable". Nobody died and nothing broke.

But life is MUCH MUCH better with the 4.88's.

Edit: In case my post is at all confusing- I am not in any way saying 3.21's are acceptable or advisable on larger tires. Merely pointing out it is possible, but not enjoyable.
 
#19 ·
So the stock tire is a 29"? So ANY upgrade Im going to have to regear first if im not mistaken. Ive seen the chart and was wondering whats still driveable. Most of the time Ill be driving up and down a hill as Im in a residential area in NY and there are hills and the winters are bad so that is why Im curious what I can do. I plan to do the bumpers and lift then the tires and perhaps regear. If I regear Ill probably get some stupid sized tire lol
 
#21 ·
The true base mode Sport tire is 29 inches, and that is what those gears were designed for. Of course Jeep wants to sell more Jeeps and most folks don't want tiny tires, so they toss on the 32s - making you undergeared off the lot.

I would reprioritize your build plan. Do lift, tires and gears at the same time, then do bumpers.
 
#26 ·
It's not that I CAN't use my 6th gear, its that its too low to use below 70.

So how is that hurting ANYONE around me? I can still hum along at 70 in 5th. With the 35's that puts me at right around 2500 RPM's. EXACTLY where I would be in 6th if I had the 4.56's.

So lets figure this out. I can still do freeway speeds. I can still do surface street speeds. I can still accelerate briskly enough that I am not obstructing traffic. WHO IS BEING HURT, AND HOW EXACTLY??

My gearing is ACCEPTABLE, but not optimal.
 
#28 ·
Agree to disagree.

I'd still toss you a tow strap if needed.

=^)
 
  • Like
Reactions: Cummins_Powered
#31 ·
@Fixmy59bug 3.73/6spd/3.6 and my setup was acceptable for me as a daily driver.
I have 35" ko2's measured to 33.5" (now), and about 500+lbs of additional weight.
I was able to use 6th from 45+ at a snails pace to highway speeds.
Only places I have to manage the clutch is during slight inclines from a dead stop. Its not that bad at all.

I also had 3.21 on a 4dr 6spd w/ 3.6 and 32's. I would never purchase a 3.21 again. 6th gear was only overdrive.

I now have 4.56 on my current.
 
#32 ·
I put on some Mickey Thompson ATZ 33's (almost 33) with my 3.21 gears and the jeep (2016 sport) was more sluggish, was laboring to accelerate somewhat and the engine was below the power band. At 70 MPH on the highway I was just below 2000 RPM.
With my 4.10 re-gear the engine is at 2450-2500 RPM at 70 MPH and is in the power band and the jeep has no problem accelerating or maintaining the speed going up grades on the highway.
So much more fun to drive the Jeep now.
 
#33 ·
Your takeaway from all these responses should tell you that unless you plan on going rock climbing or serious off roading, stick to the stock rims+tire size. Your expense to make the ride effcient and perform quite the same is approx 10K rims+tires included..
 
#35 ·
I have a 3.8 6spd manual with 35s with 3.21 gears and it's fine. I do want 4.56 in the future but whoever said highway driving is a dog is nuts In my opinion. I don't even touch 6th on the highway. I'm in 4th or 5th at 2500 rpms doing 80 no problem. Now 3.21 and auto....screw that regear immediately for anything over 33s. I had a 3.8 auto with 33s and it was a dog.
 
#40 ·
Here is a homework assignment for all you newer folks asking this question. Before you even consider bigger tires with 3.21 gearing, pay your local Jeep dealer a visit. Ask them to test drive one of their mallcrawler builds - nearly every dealer has them, where they slap on a lift and 35s and leave the paltry 3.21 gearing. Make sure your test drive includes a freeway stint that allows you to at least briefly hit close to 70mph. If you make 70 mph, you are turning a measly 1791 RPMs.

Then ask to test drive a bone stock Rubicon with 4.10 gearing. Take that same test drive. Once you hit 70 mph, you will be around 2502 RPMs. Yes - a 30% ish increase. Nearly a full third more!

The difference will be obvious and SIGNIFICANT. That difference is why you regear. When you put bigger tires on, you want to be close to how that stock Rubicon performed, around 2500 or a bit higher, maybe slightly lower, but NOT 30% lower. This is where your engine is happily doing engine work, your transmission is happily doing transmission work, and your gearset is happily doing gear work. This is where the 3.6 engine and transmission operate their best. Being undergeared, your gears are not doing enough gear work - so your engine and transmission both have to do more of their work to pickup for the slack. They are working much harder. They are much less happy. And if you offroad being undergeared - they are downright miserable.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top