TOTM: Gear Tech 101

    Gears are made up of 2 main parts: Ring gear, and Pinion Gear. The pinion gear is what your driveshaft spins. This in return rotates the ring gear which is bolted to the carrier (differential) and that turns the axles shafts.


    Gear Ratio's
    Gear ratio's are a numeric number, like 3.73:1. This refers to how many revolutions the pinion performs before the ring gear makes one complete revolution.
    So using 3.73:1 gears as an example, the pinion would have to turn 3.73 times before the ring gear makes one full turn.
    That means that the higher the number, the lower the gear. The lower the gear means you will go slower at higher RPM’s. This is desirable offroad as it provides more torque, and more control over the throttle. It’s similar as to why we use 4-lo when off-road.



    The formula to figure what gears you need:


    Axle Gear Ratio X MPH X 336 X Overdrive / Tire Diameter = RPM
    Example:
    Stock Wranglers typically have 3.07 gears, so with stock 29” tire:
    3.07 x 55 x 336 x .8 / 29 = 1565 rpm. Now add 33’s to the jeep
    3.07 x 55x 336 x .8 / 33 = 1375 rpm Now swap for 4.10s
    4.10 x 55 x 336 x .8 / 33 = 1837 rpm


    Front and rear difference:


    Whatever gears you run, the front and rear must match. Now not all axles have exactly the same gear ratios available, but as long as they are with in 1% for road use, and 2% for off-road only use it will be ok.
    1-point difference in ratio is equal to 1%. To find the percentage difference in ratios it is necessary to divide, not subtract. In order to find the difference, divide one ratio by the other and look at the numbers to the right of the decimal point to see how far they vary from 1.00. For example: 3.54 ÷ 3.50 = 1.01, or 1%, not 4% different. And likewise 4.11 ÷ 4.09 = 1.005, or only a 1/2% difference. These differences are about the same as a 1/3" variation in front to rear tire height, which probably happens more often than we realize.
    Quick Reference
    (Based on 6cyl Jeep, for all purpose use)


    Tire Size (aprox) Gear Ratio
    31” 3.73
    33” 4.10
    35” 4.56
    37” 4.88
    39.5” 5.13

    http://www.tyres1.com has interactive calculators to help figure out speedometer correction from going to bigger tires. Your gear ratio by using the number of teeth on you ring and pinion. It converts Metric Tire sizes to Inches (ie 215/75/16 = 28.7). It also has a calculator to tell you what new rear diff ratio you need to get back to factory specs after going to a bigger tire size. The numbers they give you to take your mechnical advatage back to factory specs. These gear ratios may not always be available but you will have to find out what you can buy for your front and rear axles to get them close and decided if you want it to be higher or lower. than what was suggested



    edited to make my rambilings a little more clear......and to add the sight it was missing for some reason

    this should be Gears only. no alxe, tire, locker, ect.
    those will all be topics in the future.
    Now, lets get to discussing gears!

    Tony is talking about what ratio of gear you need to go with if you already know what size tire you will be running.


    I also think that 456's for a 35" tire is a little to low. It would be great if you have an overdrive. I know that is what it calls for but speaking from experience it just revs the engine to high. If people read this for the info it couldn't hurt to have opinions about what people think about there gear size and setup.


    How about discussing the different nomenclature of setting up gears. Like backlash, tooth contact, pinion depth, shims, crush sleave and so on. Some people may have never heard of some of these names that are part of the gear assembly. Sorry I don't have the time to look them all up for you maybe someone else can chime in and help.

    it seemed that no matter who i talked too the gold standard for regearing a tj w/35" tires was at the 4.56 gear ratio. but all the info i had seemed to show that 4.56 would just get you into the bottom of the optimum powerband for all-purpose driving. 4.88 gears would truly place you centered in the optimum range - optimum being what my jeep ran at when right off the dealer's lot.


    i think the reason for people seeming to like the 4.56 so much when running 35" tires is this - most people up the lifts, then tires, and then run it like that for awhile as they save up money for the gears. after running 3.07 or 3.73 for awhile on 35" tires the bump to 4.56 seems like the perfect match - all the while still making your normal powerband sit just shy of factory.


    i based my gearing upgrade to 4.88 with 35" tires in mind. but since i went up to the 37" tires i've noticed that i'm about where most people seem to be. meaning - it will get me where i want it to go on the highway but it just doesn't have the power that my jeep came with off the lot. i guess what i'm saying is that i would/will always try to shoot for the lower gear choice - no matter what others say. history shows that people that lift it once will most often lift it again - bringing on the need for larger tires. sucking up the high rpm's for awhile until you up the tire size again doesn't seem like all that bad of a deal compared to being under powered forever after that. besides, my experience has shown that having the rpm range slightly lower at highway speeds in fact makes your gas mileage worse. the engine is working hard to try and sustain that freeway speed. but when you run the rpm's higher - it may be revving alot...but not requiring alot of work to keep the speed up. that make any sense?


    anyways, when aaron was gearing my axles we looked at the tooth engagement of the 4.88 pinion to ring on the D30. sure, it wasn't a whole lot of surface area - but it still seems to be doing fine for me. i only have around 6k or 7k miles on them - but no ill effects so far. alot of people say that 4.88 is too low for the D30 (such a physically small gear size) but in all honesty i haven't seen lots of examples of people at my level of rock-crawling who've turned the gears to christmas tree tinsel. everyone seems to preach how it will happen - i guess i'll just face that if it does. for now - my stock motor and stock tranny seem to do okay with that gear ratio in the D30 - but my stock axleshafts may be another TOTM altogether :)
    just my 2 cents...
    chris

    Basics first?


    Gear Ratio. A calculation of the number of teeth on the ring gear (which drives the axles through the differential gears) divided by the number of teeth on the pinion gear (which is driven by the driveshaft) So if you have 11 teeth on the pinion and 44 on the ring gear, the pinion turns 4 times to rotate the ring gear (and tires) once, a 4:1 ratio.


    Pinion Gear. The gear that is driven by the driveshaft. In relation to the Ring Gear, it is smaller and is the driving gear in the set. Adjustments to the pinion gear are pinion depth and bearing preload. the pinion is supported by 2 bearings. The inner pinion bearing is usually the larger of the two and is a press fit onto the pinion. Depth is usually adjusted by selecting the correct size shims. Preload can be adjusted by either using shims to "squeeze the bearings together" or by crushing a small metal sleeve the "crush sleeve" to tighten the bearings.


    Ring Gear. The large, ring-shaped gear that is driven by the pinion gear. It is bolted to the differential carrier, which turns the differential gears, which turn the axles. Adjustments to the ring gear involve bearing preload, and moving the entire carrier left to right. On most axles encountered in Jeeps this is done by selecting the correct size shims. (some axles use large nuts to make these adjustments)


    Correct me if you see anything wrong so far...this is all off the top of my head Ill work on finding some pics for visuals

    All of this info is great hovever people with automatics should disregard it. According to the math 456s and 35s will put you over 3000 rpm at 65mph. For a Six cylinder that is screaming. Im going with 410, i will let you know how it turns out.

    2000 Sahara, All custom...
    2000 Cherokee Police package, Beater....


    I may be slowest and the stupidest

    auto's are a diffrent story, depending on if you have overdrive, and what ratio the OD is.
    and 4cylinders are diffrent as well, they need to be at a higher RPM to be in the powerband.


    so far this is very good stuff. lots of options and opinions

    Quote from "bmf24"

    not sure what tony's ramdom typing above is, but this should be Gears only. no alxe, tire, locker, ect.
    those will all be topics in the future.
    Now, lets get to discussing gears!


    the only thing my ramblings address are what the caluclators on the tyres one sight do for the most part I havent given my opinon. personally I hate to see people by all the hype eng upgrade gizmos and still not be happy and then end up regearing anyway which is all they really needed to do in the first place.


    when you put bigger heavier tires on your vehicles you need the gears to get your mechanical advatage back. the calculators i provided alow you to figure out what you need to get your vehicle back to where it was fromt he factory.


    with all that said I may have screwed up when I bought the stuff for my jeep and bought 3.73s. I figure with my non overdrive transmission and chevy v8 my highway perfoemance will be fine and the granny low will make me happy offroad. we will see what happens when I get it off road.

    how do gears differ for a 4 cylinder i am looking to have 32s by the end of summer and i am wondering what gears i shoudl be running i come stock with 410's but i want to know if i should start saving for different gears or if i am ok to run those

    bLaTaNt DiSrEgArD<br /><br /><br />TJ 98 4 banger gone but not fogotten<br />WJ 02 limited- Do all vehicle &quot;QUICK THROW A LOCKER AT IT!!!&quot;

    4cyl motors are usually geared lower to start. plug in what you want into the forumula in the top post and see what it comes out to be.

    told me 4.53 gears is what i need for 32's so to run at my best performance i need 4.56 gears is that right?

    bLaTaNt DiSrEgArD<br /><br /><br />TJ 98 4 banger gone but not fogotten<br />WJ 02 limited- Do all vehicle &quot;QUICK THROW A LOCKER AT IT!!!&quot;

    Or 4.88s There is a chart I'll try to find it.


    but remember the front and rear must match!
    or you will blow your transfer case.


    your earlier post made it seem you wanted to put a diffrent size gears front to rear.

    i didn't know what sizes i need Bmf24 told me i was crazy to think about putting 4.88's in my jeep but i am not sure how the gears work for a 4 banger i think that the 4.56 would be a good match for me i am looking to have a good combination of on-road off-road performace and i still want reasonable gas mileage i konw that once i bought a jeep gas mileage went out the window but i still like to try to keep it down.

    bLaTaNt DiSrEgArD<br /><br /><br />TJ 98 4 banger gone but not fogotten<br />WJ 02 limited- Do all vehicle &quot;QUICK THROW A LOCKER AT IT!!!&quot;

    i didnt say you were crazy for putting 4.88's in, you had it written as diffrent gears in the front and rear.


    4.56 would be a good mix of on and offroad performance. it would still crawl very well, and have the power to move.
    and at the same time you would be able to still keep up with trafic on 95.

    I run 4.56's with 35's, auto tranny and 4banger. It aint a pretty sight getting to Paragon, but once I'm there she crawls like a champ. :up: I'm right around 2900 rpms at 60mph. That gearing was just about perfect with my old 33's.

    I understand the ratios, and getting the RPMs back where they should be at highway speeds, and so theoretically the gas milage would be the same with bigger tires and corrected gearing.
    However, friction in the rear changes and the axles turn slower, as well as tire rolling resistance. I wonder if you lose or gain MPG with larger and wider tires, inflated properly?
    Also think it takes less horsepower to break you rear and/or axles when you increase the ring/pinion ratio, because they do more work than before.
    Interesting stuff, yes?

    I think but wont sware to it that the reason that the lower gears break easier is because the ring gear teeth are thinner/smaller trhan on the higher gears this is one reason that I went to and decided to stay at 3.73. hoepfully the v8 will help with my street performance

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