Chevy Nova Forum banner

Can someone explain rear end gears to me?

7K views 34 replies 20 participants last post by  vondur 
#1 ·
I keep hearing people talk about rear end gears and tossing out numbers like 4.10, 4.33, ect. or 12 bolt and 10 bolt...what does it mean!?

I ran a quick search but i wasn't sure how to search and it came back with nothing so forgive me if this has been asked before...just trying to expand my knowledge. :D
 
#2 · (Edited)
The 4.11 or 3.73 number is actually the gear ratio. More correctly it is 4.11:1 or 3.73:1. It means that driveshaft turns 4.11 times for every 1 turn of the wheels.

The pinion gear is what the driveshaft attaches to. The pinion gear is the smaller of the 2 gears. The pinion gear turns the ring gear. The ring gear is the larger of the 2 gears and turns the carrier (aka differential) which turns both axles and wheels. You determine the ratio by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear.

For example:
- a ring gear with 37 teeth and a pinion gear with 9 teeth is a 4.11 ratio (37/9 = 4.11111)
- a ring gear with 41 teeth and a pinion gear with 11 teeth is a 3.73 ratio (41/11 = 3.72727)
- a ring gear with 41 teeth and a pinion gear with 12 teeth is a 3.42 ratio (37/9 = 3.41666)

However, the ring rear is matched to the pinion. So a 41 tooth ring gear designed for a 11 tooth pinion will not work with a 12 tooth pinion (and vice versa).

The 10-bolt or 12-bolt means the number of bolts attaching the ring gear to the carrier. MOST rears have the same number of bolts on the rear cover but that's not how you tell. Some 8.2" 10-bolts (in Oldsmobiles) have 12 cover bolts. They are NOT 12-bolts.

More numbers: 8.2" and 8.5" 10 bolts refer to the diameter of the ring gear. An 8.2" 10-bolt has a ring gear with a diameter measuring 8.2" and has 10 bolts attaching it to the carrier. An 8.5" 10-bolt has a ring gear with a diameter measuring 8.5" and has 10 bolts attaching it to the carrier. A 12-bolt rear has a ring gear measuring 8.875" in diameter and has 12 bolts attaching it to the carrier.

In the same way, a Ford 9" has a 9" diameter ring gear. The 8.8" Ford has a ring gear of 8.8" in diameter and has been called a 12-bolt clone because they are very similar in design. The larger the ring gear, the stronger the rear (for the most part but there are other things that make one rear stronger than another).
 
#4 ·
That was a great explination. I just hate it when I spend a lot of time typing an answer for someone and by the time I hit submit someone else beat me to the punch and said the same thing I just said. Damn all that time typing for nothing and the first guy got all the glory. Luckly for NovaResource that didnt happen in this case.
 
#7 · (Edited)
It depends on your engines power band. If your engine makes peak power at ~7000 RPM's, then you probably don't want to stay down in the lower RPM range too often. My cars power peaks ~7500, but that is untested/dyno'd yet, and I run 4.11's with 275's. It puts me going 30 MPH at around 2500 RPM in second. Needless to say, it pulls HARD in first though.

Think of rear gears as torque multipliers. It multiplies torque received from the transmission, to the differential, to the rear wheels.
 
#8 ·
A high numerical gear will allow the engine to rev faster. A 4.88 or 4.56 gear is common in some drag cars, depending on their setup. That being said, you run out of engine at the end of the track or going down the highway.

With 2.73 gears, I could go 65-70 mph at about 2300 rpm. With 3.73's, I'm at about 3000 rpm or a little higher. This puts increased wear on the engine, more fuel consumption and the exhaust is louder. Bad for daily driving.

You also run out of top speed. A car at WOT with "steep" high numeric gears (4.88 ie) will tend to push the motor really hard at the end of the track. It might hit 7800 or 8000 rpm with a fast engine. If your motor doesn't make power up there, you essentially hit a wall. Your valves start floating and the engine wants to come apart. The idea is to get a gear that will allow you to accelerate the fastest without pushing the engine beyond its operating range. On the street, it's better to compromise towards to 3.42/3.73 gear sets to make highway driving livable.

Kev
 
#9 ·
Right..If your rear end is a posi..( both Rear Tires turning at equal torque...say....4.56...You'll feel as tho you need a 5th..6th gear to bring the rev down....With a lower gear ratio....3.73...you can drive comfortable on the highway in 4th gear not having that feeling of needing another gear....your engine won't rev so high....But you still have plenty of torque applied to both rear tires getting off the line...just not as fast...
 
#13 ·
Just so that i understand what your saying here, when you say 2.52:1, is that saying the tranny is spinning 2 1/2 times to the driveshaft once or vice versa?
Correct. the input shaft of the transmission will spin 2.52 times to 1 rotation of the shaft. consider this, that a lower first gear can have the same effect as lowering your axle ratio. the first gear in a TH350 is 2.52:1, and in a 700r-4 is 3.06:1. so, the ratio of the transmission input to tire rotation is:

TH350=2.52*3.73=9.3996
700r-4=3.06*3.73=11.4138

this means that the 700r-4 has a lower input (engine speed) to output(wheel speed) ratio at launch than a TH350. so in addition to being able to run lower gears and still drive on the highway (because of the overdrive), you also get more torque multiplication off the line.
 
#16 ·
Tall gears ( 4.11:1) are slow. You use slow gears to go quick.

Short gears ( 2.41:1 ) are fast. Fast gears are slow 'out the gate'.

I had a 2.41:1 on my 78 firebird. It was awesome on the hiway. I could dump it into first gear at 45 MPH and grab a car length instantly. Could leave it in second till near 90! Cruising at 60, I could dump it into second, and pass someone easy.

And I really gotta make a comment on the afore mentioned pozi comment..

Whether or not the rear end is positive traction, either locker, spool, clutch type... has no effect whatsoever on gear ratio. It only effects a tire which is slipping. At speed, a pozi rear feels just like an 'open' rear with exception to the fact that the 'open' rear does not 'pop' when going around turns.

It really surprised me that so many replies didn't catch that..

A little explanation for the dude whom posted the original question: and more that you never knew, and didn't want to learn:

An 'open' rear end has nothing to stop one wheel from spinning. If you loose traction on one side, that wheel will spin, and the other will not. All power will be effectively transferred to the spinning wheel. Not good for getting off the line.

Drag racers prefer a 'pozi' rear end. There are many designs. The detroit locker POSITIVELY LOCKS ( pozi ) the axles together under load. one side cannot slip. There are variations of the locker design. Most engage with a noticable CLUNK. Some can be engaged manually, most function automatically. Most are not street friendly.

Then there are limited slip.. these are often referred to ( incorrectly ) as pozi rears. Limited slip is also varied in design, but mostly they use either cones or clutches to create friction between the axles. They CAN turn independently, but must overcome the friction of the clutches/cones to do so.

OK. so WHY would one want the rear axles to turn independently? Well, if you are driving your car in other than a straight line, it is highly desireable. Why? because in a turn, the outside wheel is moving faster than the inside wheel. Now, if the axles can't turn independently, the outside wheel would 'hop' through the turn. If you have an open rear, as do most passenger cars, ( and my lawn mower ) your turn will feel as smooth as a straight line. BUT.. if you get in the mud ( or wet grass in the case of my mower ) one tire will inevitebly have SLIGHTLY less traction than the other. That tire will spin, and you will not move.

ENTER LIMITED SLIP.. I had a limited slip in one of my firebirds. The clutches were set to about 40 ft/lb. That means, that for the axles to spin independently, there would have to be over 40 foot/pounds of torque differential. This accomplishes several things... When I was stupid enough to put one wheel off the shoulder on my way to california, in the mud.. I was able to just drive away. Had I not installed the limited slip, then I would just spun a tire, and dug into the mud. Another thing I noticed limited slip does for you.. it wears out the inside of the rear tires. RAPIDLY. That rather sucks. And, my particular limited slip, would 'pop' when going into a turn, if I had run in a straight line for a while ( think interstate ) This is due to the gear oil getting slung out from between the clutch discs, so they get real good traction on each other. Then you go into a turn, and they 'pop' loose. Annoying. There are additives you can put in the gear oil which will help with this, but I didn't notice that they actually helped. And finally.. the real reason I put in the limited slip.. the car had a tendency for the rear to move sideways when I punched it. I thought the limited slip would eliminate this.. it actually made it worse.

One last nugget.. the 'spider' gears are what allow the axles to actually move independently of one another. An old dirt track trick ( cheap hot rodding ) was to weld the spider gears, so they couldn't move. Then, so the car wouldn't be 'hopping' through the round track, put a larger tire on the outside wheel.
 
#20 ·
I am going thogh in a $.02 here as well... and speak to the POSI issue. NovaResource is dead on with his vernacular of the orgins of the term POSI,..... Mopar called their LSD SurGrip, and AMC called theirs..TwinGrip..and Dana called theirs SafeTgrip ..... all Clutch and preload spring based units....

there is one there LSD..... a Thorsen, this unit has no clutches or a ratcheting type lugs/cogs action of the Detroit locker( locker), the Thorsen differential uses Helix cut worm style gears to to create a bind action by using a torque value to over come slippage....Eaton sells the Detroit TruTrac which is the Thorsen style, as well as the Detroit Locker....

as for gears... NovaResource did a Fine explanation.. I would add only this.....

gears with a High numerical value IE: 4:11, 4.56, 488, 5.13, 6.30 are call low gears even tho they represent a higher numerical value... and the preceding explanations By NovaResource of the use or how they function is dead on...

consider this.. as explanation as well... Heavy loads...... by raising the numerical value of the differential a heavier load can be moved with less effort...with a gear of 5.0:1 as apposed to a gear of 2.50, it effectively reduced the load strain by half.... so moving a 20 ton load with a 250 gear would require 100% effort, where as the 5.0 gear needs only half the effort to get the load moving.....

But in raising the numerical value of a differential you slow top speed or..... you are limited in top speed by the Max RPM of your engine...

Hope I have added to the confusion:D

Veno
 
#21 ·
.

Good thing i did some reading on Howstuffworks.com about gears otherwise when i read this post i would have been lost right abooout here...

...uses Helix cut worm style gears to to create a bind action by using a torque value to over come slippage....
Veno

And if anything veno you brought some clarity to the subject. Thansk to everyone's contribution i do have a much better understanding of the method behind the madness.

Once again so that im understanding...would you guys say a larger rear end such as 4.11 be a better choice for a straight drag strip because the larger rear end can push the car off the line faster and with less effort than say a 2.41? With that larger rear end you trade top speed for better shot off the line? and then of course a smaller rear end makes sense for street driving, no explanation needed.

and finally...are there set number for rear ends, a pattern? or do they vary depending on manufacturer? the best example i can use is a socket set, when you buy a SAE socket set or metric set, they always have the same sizes starting from smallest to biggest...the system does not vary by manufacturer meaning there is no 10.25mm socket, it goes from 10 to 11.

Example: (just #'s off the top of my head): 2.50, 3.00, 3.50, 4.00, 4.50, ect...
 
#22 · (Edited)
If you are using your car solely for the drag strip, you need to determine what your top speed will be at the end of the 1/4 mile (or 1/8th mile). You need to make sure your rear gear, tire size and engine RPM will get your to your desired top speed.

If you are building a dual purpose car for street and strip, you will have to find a compromise. 4.10 is about as low as I'd go for a street driven car, but there are people running lower, especially if they have 28 inch tires in back.

Edit:

Here is a good gear calculator.

Assuming your trying to get 125 MPH with a 7000 RPM redline and a 28 inch tire, you will need a 4.56 rear gear and a very tight torque converter or a manual transmission. If you have a 6500 RPM red line, you will need a 4.10 gear
 
#24 · (Edited)
Thanks for clearing up my pozi mispelling, and clearing up the vernacular.

Another tidbit which occured to me, and I'm not sure was addressed ( and I don't want to re-read the whole thread )

You CANNOT swap pinions and ring gears. They are made as a set.

Before you decide to drop in a new set of gears, be sure to learn how to set them up. The pinion gear has to ride in the center of the ring gear. To accomplish this, the correct shims and crush must be set. ( I'm not totally clear on this myself, I let a qualified mech do it for me ) I do seem to recall however, that you can easily check the pinion setting with chalk. You chalk up the ring gear, and spin it. The resulting pattern from the pinion will be evident, and you can see if the gears are meshing to center, or off-center.

Oh one note to Nova man.. don't call the 4.11+ larger, call them TALLER. And conversely of course, shorter. Larger rear ends would refer to the 10-12 bolt such as the DYNA ( the holy grail of rear ends ) which are simply.. physically larger.

clear as mud huh!
 
#25 ·
Some more fill ins.
3.08 vs. a 4.11 gear. Because of the gear teeth, cuts etc. In the same size rear end, the 4.11 is usually weaker. So the bigger ring gear can hold more gears and be strong enough. A 4.11 in a 9" is a lot stronger than a 4.11 in an 8.2".

Strong and weak is relative. My rule of thumb is the stock 8.2" rear ends are usually just fine with 350 HP. 500HP, you better be looking at a stronger setup in the rear end itself and a new mounting method to the car in a Nova.

To get a 5.13 gear the pinion gear angle gets extreme. I can often hear such a rear end "whinning" where a 3.73 is silent.

When disassembling a rear end, if you are going to re-use the gears you need to see if it is hunting or a non-hunting gear set. (That is what it was called many years ago in the Auto mechanics class)

Simple examples: When the Pinion has 4 teeth and the ring has 12, this is a non-hunting gear set. The pinion teeth always mesh in the same teeth in the ring as it turns. If you re-assemble it and don't get the same teeth meshing again, it could make noise/more noise. Noise is often a result of fine variances in the teeth alignment or wear patterns.

If the pinion has 4 teeth and the ring has 13 teeth, this is a hunting system. As the gears turn, the pinion teeth will run in all the teeth in the ring after a few turns. Re-assembly takes no special precaution for aligning the teeth.
 
#26 · (Edited)
Nice

Great post, I learnt alot.

I recently bought a 64 Nova with a 305/TH350. The rear gear ratio is way too low. Highway driving is uncomfortable and 1st gear runs out way earlier than necessary. Possibly the car has been used with other tranny and engine in the past and was geared down (up?).

Anyway, the thing I want to know is what rearend gear ratio should I aim for? Whats standard? Its hard to guesstimate for me since I dont know what ratio Im currently running and just like OP I didn't even know what the numbers meant until I read this thread.

Oh, and whats the easiest way to change it?

Thanks
 
#27 ·
Great post, I learnt alot.

I recently bought a 64 Nova with a 305/TH350. The rear gear ratio is way too low. Highway driving is uncomfortable and 1st gear runs out way earlier than necessary. Possibly the car has been used with other tranny and engine in the past and was geared down (up?).

Anyway, the thing I want to know is what rearend gear ratio should I aim for? Whats standard? Its hard to guesstimate for me since I dont know what ratio Im currently running and just like OP I didn't even know what the numbers meant until I read this thread.

Oh, and whats the easiest way to change it?

Thanks
First thing I would do is raise your car.....SAFELY!! and crawl under it. Make sure the car is secure and have some friends around to help. With the trans in neutral turn the drive shaft while counting the turns until the rear tire makes one full revolution. This difference between the drive shaft and the tire is your rear gear ratio. It would probably be easier to replace the trans with a 700R4 and get the benefits of that rear gear than to toss it out. Just my 0.02 cents.
 
#28 ·
If you've ever ridden a bicycle with more than one speed, you know how different gear ratios effect speed and torque.
 
#34 ·
What a great (old) post, I missed it.

Everyone pretty much nailed it on ratios, spot on..

Now, for my 2 cents. Its a system, the car, every part of it. You can look at individual parts of the system, but try not to get too focused on the individual parts, they work TOGETHER as an entire system. An understanding of the workings of each part of the system is needed.

Then, once you get the idea of how each component does its job the difficult part is to incorporate each of these sub components into the entire system, the car as a whole.

And thats the tuff part. All the auto manufactures pay thier engineers pretty good money to make the components work together as a compleate system, the car on the show room floor. Its a sacrifice, longevity and performance with a low price tag. I dont want that job!!!

It amazes me when guys do get it right. And those are the ones that are winning races (if its a racing car) or the ones that have a dependable and usable cruiser (steet car).

So now that the rear end gear ratio is understood. And how it interacts with the transmission gearing now what??

And Ill just talk about a street car cause really. The racing cars are just too varied in ALL the possibilities. And each car should be looked at individually, you cant say what works here on this car will work on another. Racing cars are completely independent of each other racing car, thats one of the reasons we have winners and losers. They are as independent as the driver sitting in the car next to them.

But street cars have more commonalities. You can pull up to a similar year and make car on the road and unless its a racing car on the street it still has alot of similar aspects of your car. Sure, it may have independent suspension, big block, fuel injected, yadda ya. But if its truly a street car, able to travel a few hundred miles up the road and back without AAA help then its still held within the limitations of a street car.

So with that..... Final rear end gear ratios are IMO the last thing that should be looked at. Why? Well, its one of the easiest of the drive train components to change.

Just my thoughts but there are several things to consider before picking the rear end gears, for a street car.

You cant change the weight of your car that much, so final weight of the car is a biggy. Horse power and torque of the engine. If you have a stump pulling BB that loves to give the torque down low then you can get away with a shorter gear, and you should use a shorter gear. Otherwise you are wasting that torque monster on a tall gear that wont push the engine. It never gets to come even close to its torque curve, you wont ever get there.

But on a nippy mouse engine, that doesnt have any torque down low but loves to show its muscle up a lil higher in the RPM band then a tall gear will help. An engine lacking on torque will love a tall gear. The tall (fast) gear allows the small mill to get into its higher RPM band to give some serious performance.

So now what, I have a mighty mouse and tall gears but Im running at 3000 rpm on the freeway just to keep up with the 65MPH crowd.. Well thats where the transmission is supposed to pick up the slack. Give the mouse a break once she is up to speed.

And so GM and all the others came up with the OD transmissions, auto and manual. Kick down the rpms alil at highway speed, where the car is already in motion, doesnt take much to push the block through the air once she is moving, toss in an over driven gear ratio. Yea!! Problem solved. Well kinda. They are a problem with carburators, the carb would go into idle if the driver slowed down to 55mph and they lost power and had to give more pedal. Tranny kicked down and brought the Rs back up for the carb to drink. Back and forth, wasnt good.

Then EFI was a perfect match, sorta. The EFI could cruise at 1800 rpm on the highway, a slight dip in on the throttle to keep a constant speed wasnt an issue, it would adjust timing and fuel. But we kinda lost control. Sensitive control. We were held back by the computer. Give it a slight tip in on the throttle and it would just kinda bog, trying to stay in the top gear. So we tipped in further and further till it down shifted. And on some cars its abrupt.

Make it worse, now we have drive by wire. I hate that. Im always fighting the throttle (computer) for where I want the power to be applied. Ummm?? I HATE my drive by wire cars I have, they drive me nutts.

OK... I digress........... Weight of the car, torque of the engine, size of the rear drive tires, transmission ratios and really, the needs of the car will determine what your rear end gear ratio will be. A BB with 600lbs of torque in a 1000lb car can live with a 2.73:1 rear, it will smoke the tires, no matter how wide. A 4000lb car with the same engine might like to have a lil taller gear to smoke the same tires :) JR
 
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