Why Regear?

Hey fellas, i was wondering if you knowledgeable fellow jeep guys could help me understand why it is essential to re-gear after moving up to bigger tires. I'm feeling kinda like a noob right now because i don't think i am fully understanding it. Why do you re-gear? What is the point? Also, i was told that if i re-gear i would be losing my top end, in which i would max out at 65 mph. With the re-gear, will i still be able to go 70 mph? What do the gear ratio numbers mean?

So by the end of the summer i plan on buying either a used rear d44, or a refurbished ford 8.8 from ECGS. Now with my plan to move up to 33's i will be looking to re-gear seeing as it will be my daily driver. What gear ratio is best.

I know i could find a topic about this either on jeep-forum or Google, but i thought i would post something here on our forum. Plus, it will be good to have a topic like this on here.

Thanks guys

Andrew, you don't need to regear for 33's. A lot of people say that you lose gas mileage and power if you don't, because the wheels don't rotate as fast as they should with stockers due to their increased diameter and weight. Personally, I haven't found that to be the case with mine. My speedo gear is a bit off, but that's easy to compensate for/change.
Here are some good resourses:
Calculator

The best chart out there

Think of a re-gear as changing the gears on your bike. When you're on a flat road, or a downhill, you shift to a larger gear in order to go faster. However, when you hit a hill you downshift in order to accommodate the change in terrain. When someone puts 35" tires on 3.07 gearing, it's essentially hitting that same hill but WITHOUT shifting to a more reasonable gear. Your legs burn, you fight the pedals, and you get tired quickly.

Re-gearing a Jeep is basically downshifting the bike. Your gear ratio is adjusted for bigger tires (in this analogy, the steeper hill) in order to maintain a proper situation for the engine (or your legs).

The "JeepForum" default is as follows:
31" Tires - 3.07, 3.73, or 4.10
33" Tires - 3.73, 4.10, 4.56
35" Tires - 4.10, 4.56, 4.88

I can explain more as needed, but maybe this will get you started in understanding. For 33s, if you've got 3.07 or 3.73, you will be fine without a re-gear. Save your graduation money.

Warning: verbose and "educational". I apologize for "duh" statements. They're not meant to be patronizing or anything. I just figured I'd try and cover all bases.

To expand on what Ryan said, the best way to get the reason behind it is to understand what it means.

A 3.07 gearing means the driveshaft turns 3.07 times to get the tires to spin one rotation.
A 4.10 gearing means the driveshaft turns 4.1 times per tire rotation.

Say you have to push an object 10 feet. It would take more effort per push to do it in 3 pushes than it would to do it in 4 pushes.

So that's the biggest reason, is that your engine doesn't have to work as hard to spin the tires. You accelerate more slowly (because you have to have more rotations to move the tire), but the engine doesn't have to work as hard to push the tires.

To explain the top end and make it even more confusing:

Hold tight: I'm going to attempt math.

Circumference of a tire is calculated by the diameter x pi (3.14). So when you went from 28" tires to 33" tires, your distance per rotation went from 88" to 104".

That means that putting on larger tires (assuming your engine has the power) actually makes you go faster at the same RPMs (because you're traveling a longer distance per tire rotation)

When you regear, you're basically re-distributing your shift points to be more like they were when you were on the stock setup.

Example: Shifting from second gear
Stock: You hit 3000rpm at 30mph and need to shift into third gear.
Bigger tires on stock gearing: Because of the bigger tires going further per rotation, by 3k you're actually at 37mph by the time you shift into 3rd.
Bigger tires with a regear: At 3000rpm your driveshaft is turning the same as the other two, but because it needs more turns per tire rotation, the tire rotates more slowly than before the regear. But because the taller tire goes further per rotation, you end up still reaching a 3000k shift point at around your original 30mph.

So all you're doing is redistributing your shift points back to where they were stock. You don't lose top end speed from where it was stock. You're just taking back the extra speed you got from the taller tires.

Now if you've got the 6 cylinder engine, you may not feel a need to regear for 33s. Many don't.

I went to 32s and my dog of a 2.5L engine was so awful that I started having back problems from leaning forward like a race jockey trying to get up a hill, ha ha. So I actually *overgeared*.

With my 4.88s on 32s, I have a top speed of around 65mph at 3000k. More power around town and on the rocks (and recovered quite a bit of gas mileage), but that's when you lose that top speed.

So if you're going to replace axles (both), you might as well look for something in a preferred gear ratio. But if you're just worried about the daily driver, you'll be alright with the 6 cyl.

did you install appropriate speedo gear so you do not fool yourself on the mileage or speed to rpm ,

Those answers were much better than mine.

Thanks guys! I think i understand it now. So, 33's on 3.07's should be ok. But if i find an axle with 3.73's ill do the swap and change the front diff to match. Maybe ill think about going to 35's. Thats gonna be a stretch though, i might save money in the long run. Though i think 33's will be big enough for right now.

33s with 307gears is No Good. You will have no power at all , especially on highway.
The same with 35s with 373 is NG..
31s with 307s will work.
32s is pushing it.
I have a 97 tj , 4.0, 5 speed, 33s with 373s and i do not have nearly enough power.

I wouldn't waste your time.
You should use at the minimum 410s with 33" tires.
You can get a front & rear diff out of of a 4 Cyl TJ. They have 410s as factory gearing.
You could also get Dana 30 front end in a XJ with 410 gearing if you look around.

Amills60 wrote:Thanks guys! I think i understand it now. So, 33's on 3.07's should be ok. But if i find an axle with 3.73's ill do the swap and change the front diff to match. Maybe ill think about going to 35's. Thats gonna be a stretch though, i might save money in the long run. Though i think 33's will be big enough for right now.

Clayton ran 33s on 3.07 for a few years before ever switching to 4.10. It's doable, although not IDEAL, it is absolutely doable.

That's what I've been running for 2 years now, without a problem. The only time I notice a lack of power is going up steep hills on highways, such as 2 north coming out of Boston. That thing kills me daily.

Not that there is a problem it's just that power thing.

I know its a world of difference with the stick, but my auto blows with 33's with the 4.10's on the highway hills. Can really feel the lack of power and my speed drops.

Even still I could only imagine the lack of power with 3.07's even with a stick... You can always try it out, see how you like it/ try deal with it, before shelling out the coin for a regear, as thats kinda what I'm doing with mine. Not bad enough to warrant the $$ at the moment, but once I go bigger, it'll change.

I used to have a 98 TJ with the 4.slow and a 5spd. D30/D35, both locked, with 3.07's and 33's, either the LTB's or the BFG MT's depending on what day it was.

I drove and wheeled this set up for about 4 years. So its definitly doable. There is a noticble lack of power on inclines though. And you can forget about 5th gear unless you want to go 75. lol I dont think I ever used it, or used it rarely.

In the woods, its also noticeable somewhat. In places I used to be able to use only 4high, i had to use 4low now with the bigger tires and 3.07's.

Again... is it noticable? Yes. Is it doable? Yes.
I say the same thing, do it and try it and see if its acceptable to you. Not everyone is the same, just like every jeep isnt. Whats acceptable to me, may not be to you. But personally I think you'll be ok for a year or two while seeking out some new axles. Marks suggestion is a good one. Seek out the HPD30 with 4.10's, and a rear 8.8 or 44 with 4.10's already in them.

I know I'm entering this conversation REALLY late, but I saw Ryan brought me up, and I feel like I should comment. Even though I didn't regear after going to 33's, I was still able to drive around and get to school and such. However, as soon as I went off road with it, that's when it started to suck.

Since the gears were too high, my shift point on my clutch was really close. So close as in I was burning out my clutch because my engine and transmission had to compensate for my lack of proper gearing. This eventually led to clutch failure, and thus my clutch was replaced. I essentially smoked it out because of the lack of power in the gears.

Now that I'm on 4.10's, it feels great. I probably get around 18 on the highway, and I still have plenty of power going up hills.

Thanks for the reply Clayton! Im going back and forth between going to 33's, or skipping 33's and going right to 35's. I can't make up my mind.

If you have 3.73s or anything less go with 33s and wear them out getting familiar with the jeep.

Markmmaark wrote:If you have 3.73s or anything less go with 33s and wear them out getting familiar with the jeep.

This.

Agreed. Due to having 3.73s and not enough $$$ to regear, I stayed on 33s. If I decide to go 35s, I know there are a few other things that NEED to be done first:

HD Steering
BBK
Gears
Flat Fenders

With all that said, I think I will be on 33s for awhile.